The Atlantic Monthly, Volume 12, No. 74, December, 1863

And trims his helmet's plume,
When the good-wife's shuttle merrily
Goes flashing through the loom,
With weeping and with laughter
Still is the story told
How well Horatius kept the bridge
In the good old days of old."

The bridge of Darius spanned the Bosphorus,—of Xerxes, the Hellespont,—of Cæsar, the Rhine,—and of Trajan, the Danube; while the victorious march of Napoleon has left few traces so unexceptionably memorable as the massive causeways of the Simplon. Cicero arrested the bearer of letters to Catiline on the Pons Milonis, built in the time of Sylla on the ancient Via Flaminia; and by virtue of the blazing cross which he saw in the sky from the Ponte Molle the Christian emperor Constantine conquered Maxentius. The Pont du Gard near Nismes and the St. Esprit near Lyons were originally of Roman construction. During the war of freedom, so admirably described by our countryman, whereby rose the Dutch Republic, the Huguenots, at the siege of Valenciennes, we are told, "made forays upon the monasteries for the purpose of procuring supplies, and the broken statues of the dismantled churches were used to build a bridge across an arm of the river, which was called, in derision, the Bridge of Idols."

But a more memorable historical bridge is admirably described in another military episode of this favorite historian,—that which Alexander of Parma built across the Scheldt, whereby Antwerp was finally won for Philip of Spain. Its construction was a miracle of science and courage; and it became the scene of one of the most terrible tragedies and the most fantastic festivals which signalize the history of that age, and illustrate the extraordinary and momentous struggle for religious liberty in the Netherlands. Its piers extended five hundred feet into the stream,—connected with the shore by boats, defended by palisades, fortified parapets, and spiked rafts; cleft and partially destroyed by the volcanic fireship of Gianebelli, a Mantuan chemist and engineer, whereby a thousand of the best troops of the Spanish army were instantly killed, and their brave chief stunned,—when the hour of victory came to the besiegers, it was the scene of a floral procession and Arcadian banquet, and "the whole extent of its surface from the Flemish to the Brabant shore" was alive with "war-bronzed figures crowned with flowers." "This magnificent undertaking has been favorably compared with the celebrated Rhine bridge of Julius Cæsar. When it is remembered, however, that the Roman work was performed in summer, across a river only half as broad as the Scheldt, free from the disturbing action of the tides, and flowing through an unresisting country, while the whole character of the structure, intended only to serve for the single passage of an army, was far inferior to the massive solidity of Parma's bridge, it seems not unreasonable to assign the superiority to the general who had surmounted all the obstacles of a northern winter, vehement ebb and flow from the sea, and enterprising and desperate enemies at every point."[7 - History of the Netherlands, Vol. I. p. 182.]

Even the fragile bridges of our own country, during the Revolution, have an historical importance in the story of war: the "Great Bridge" across the Elizabeth River, nine miles from Norfolk in Virginia, the floating bridge at Ticonderoga, that which spanned Stony Brook in New Jersey, and many others, are identified with strife or stratagem: King's Bridge was a formidable barrier to the invasion of New York by land. Indeed, from Trenton to Lodi, military annals have few more fierce conflicts than those wherein the bridge of the battle-ground is disputed; to cross one is often a declaration of war, and Rubicons abound in history.

There is probably no single problem, wherein the laws of science and mechanical skill combine, which has so won the attention and challenged the powers of inventive minds as the construction of bridges. The various exigencies to be met, the possible triumphs to be achieved, the experiments as to form, material, security, and grace, have been prolific causes of inspiration and disappointment. In this branch of economy, the mechanic and the mathematician fairly meet; and it requires a rare union of ability in both vocations to arrive at original results in this sphere. To invent a bridge, through the application of a scientific principle by a novel method, is one of those projects which seem to fascinate philosophical minds; in few have theory and practice been more completely tested; and the history of bridges, scientifically written, would exhibit as remarkable conflicts of opinion, trials of inventive skill, decision of character, genius, folly, and fame, as any other chapter in the annals of progress. How to unite security with the least inconvenience, permanence with availability, strength with beauty,—how to adapt the structure to the location, climate, use, and risks,—are questions which often invoke all the science and skill of the architect, and which have increased in difficulty with the advance of other resources and requisitions of civilization. Whether a bridge is to cross a brook, a river, a strait, an inlet, an arm of the sea, a canal, or a valley, are so many diverse contingencies which modify the calculations and plans of the engineer. Here liability to sudden freshets, there to overwhelming tides, now to the enormous weight of railway-trains, and again to the corrosive influence of the elements, must be taken into consideration; the navigation of waters, the exigencies of war, the needs of a population, the respective uses of viaduct, aqueduct, and roadway, have often to be included in the problem. These considerations influence not only the method of construction, but the form adopted and the material, and have given birth to bridges of wood, brick, stone, iron, wire, and chain,—to bridges supported by piers, to floating, suspension, and tubular structures, many of which are among the remarkable trophies of modern science and the noblest fruits of the arts of peace. Railways have created an entirely new species of bridge, to enable a train to intersect a road, to cross canals in slanting directions, to turn amid jagged precipices, and to cross arms of the sea at a sufficient elevation not to interfere with the passage of ships,—objects not to be accomplished by suspension-bridges because of their oscillation, nor girder for lack of support, the desiderata being extensive span with rigid strength, so triumphantly realized in the tubular bridge. The day when the great Holyrood train passed over the Strait of Menai by this grand expedient established the superiority of this principle of construction, and became a memorable occasion in the annals of mechanical science, and immortalized the name of Stephenson.

We find great national significance in the history of bridges in different countries. Their costly and substantial grandeur in Britain accords with the solid qualities of the race, and their elegance on the Continent with the pervasive influence of Art in Europe. It is a curious illustration of the inferior economical and high intellectual development of Greece, that the "Athenians waded, when their temples were the most perfect models of architecture"; and equally an evidence of the practical energy of the old Romans, that their stone bridges often remain to this hour intact. Our own incomplete civilization is manifest in the marvellous number of bridges that annually break down, from negligent or unscientific construction; while the indomitable enterprise of the people is no less apparent in some of the longest, loftiest, most wonderfully constructed and sustained bridges in the world. We have only to cross the Suspension Bridge at Niagara, or gaze up to its aërial tracery from the river, or look forth upon wooded ravines and down precipitous and umbrageous glens from the Erie Railway, to feel that in this, as in all other branches of mechanical enterprise, our nation is as boldly dexterous as culpably reckless. As an instance of ingenuity in this sphere, the bridge which crosses the Potomac Creek, near Washington, deserves notice. The hollow iron arches which support this bridge also serve as conduits to the aqueduct which supplies the city with water.

Amid the mass of prosaic structures in London, what a grand exception to the architectural monotony are her bridges! how effectually they have promoted her suburban growth! Canova thought the Waterloo Bridge the finest in Europe, and, by a strangely tragic coincidence, this noble and costly structure is the favorite scene of suicidal despair, wherewith the catastrophes of modern novels and the most pathetic of city lyrics are indissolably associated. Westminster Bridge is as truly the Swiss Laboyle's monument of architectural genius, fortitude, and patience, as St. Paul's is that of Wren; and our own Remington's bridge-enthusiasm involves a pathetic story. At Cordova, the bridge over the Guadalquivir is a grand relic of Moorish supremacy. The oldest bridge in England is that of Croyland in Lincolnshire; the largest crosses the Trent in Staffordshire. Tom Paine designed a cast-iron bridge, but the speculation failed, and the materials were subsequently used in the beautiful bridge over the River Wear in Durham County. There is a segment of a circle six hundred feet in diameter in Palmer's bridge which spans our own Piscataqua. It is said that the first edifice of the kind which the Romans built of stone was the Ponte Rotto, begun by the Censor Fulvius and finished by Scipio Africanus and Lucius Mummius. Popes Julius III. and Gregory XIV. repaired it; so that the fragment now so valued as a picturesque ruin symbolizes both Imperial and Ecclesiastical rule. In striking contrast with the reminiscences of valor, hinted by ancient Roman bridges, are the ostentatious Papal inscriptions which everywhere in the States of the Church, in elaborate Latin, announce that this Pontiff built or that Pontiff repaired these structures.

The mediæval castle moat and drawbridge have, indeed, been transferred from the actual world to that of fiction, history, and art, except where preserved as memorials of antiquity; but the civil importance which from the dawn of civilization attached to the bridge is as patent to-day as when a Roman emperor, a feudal lord, or a monastic procession went forth to celebrate or consecrate its advent or completion; in evidence whereof, we have the appropriate function which made permanently memorable the late visit of Victoria's son to her American realms, in his inauguration of the magnificent bridge bearing her name, which is thrown across the St. Lawrence for a distance of only sixty yards less than two English miles,—the greatest tubular bridge in the world. When Prince Albert, amid the cheers of a multitude and the grand cadence of the national anthem, finished the Victoria Bridge by giving three blows with a mallet to the last rivet in the central tube, he celebrated one of the oldest, though vastly advanced, triumphs of the arts of peace, which ally the rights of the people and the good of human society to the representatives of law and polity.

One may recoil with a painful sense of material incongruity, as did Hawthorne, when contemplating the noisome suburban street where Burns lived; but all the humane and poetical associations connected with the long struggle sustained by him, of "the highest in man's soul against the lowest in man's destiny," recur in sight of the Bridge of Doon, and the two "briggs of Ayr," whose "imaginary conversations" he caught and recorded, or that other bridge which spans a glen on the Auchinleck estate, where the rustic bard first saw the Lass of Ballochmyle. The tender admiration which embalms the name of Keats is also blent with the idea of a bridge. The poem which commences his earliest published volume was suggested, according to Milnes, as he "loitered by the gate that leads from the battery on Hampstead Heath to the field by Camwood"; and the young poet told his friend Clarke that the sweet passage, "Awhile upon some bending planks," came to him as he hung "over the rail of a foot-bridge that spanned a little brook in the last field upon entering Edmonton." To the meditative pedestrian, indeed, such places lure to quietude; the genial Country Parson, whose "Recreations" we have recently shared, unconsciously illustrates this, when he speaks of the privilege men like him enjoy, when free "to saunter forth with a delightful sense of leisure, and know that nothing will go wrong, although he should sit down on the mossy parapet of the little one-arched bridge that spans the brawling mountain-stream." On that Indian-summer day when Irving was buried, no object of the familiar landscape, through which, without formality, and in quiet grief, so many of the renowned and the humble followed his remains from the village-church to the rural graveyard, wore so pensive a fitness to the eye as the simple bridge over Sleepy-Hollow Creek, near to which Ichabod Crane encountered the headless horseman,—not only as typical of his genius, which thus gave a local charm to the scene, but because the country-people, in their heartfelt wish to do him honor, had hung wreaths of laurel upon the rude planks.

Fragments, as well as entire roadways and arches of natural bridges, are more numerous in rocky, mountainous, and volcanic regions than is generally supposed; the action of the water in excavating cliffs, the segments of caverns, the, accidental shapes of geological formations, often result in structures so adapted for the use and like the shape of bridges as to appear of artificial origin. In the States of Alabama and Kentucky, especially, we have notable instances of these remarkable freaks of Nature: there is one in Walker County, of the former State, which, as a local curiosity, is unsurpassed; and one in the romantic County of Christian, in the latter State, makes a span of seventy feet with an altitude of thirty; while the vicinity of the famous Alabaster Mountain of Arkansas boasts a very curious and interesting formation of this species. Two of these natural bridges are of such vast proportions and symmetrical structure that they rank among the wonders of the world, and have long been the goals of pilgrimage, the shrines of travel. Their structure would hint the requisites, and their forms the lines of beauty, desirable in architectural prototypes. Across Cedar Creek, in Rockbridge County, Virginia, a beautiful and gigantic arch, thrown by elemental forces and shaped by time, extends. It is a stratified arch, whence you gaze down two hundred feet upon the flowing water; its sides are rock, nearly perpendicular. Popular conjecture reasonably deems it the fragmentary arch of an immense limestone cave; its loftiness imparts an aspect of lightness, although at the centre it is nearly fifty feet thick, and so massive is the whole that over it passes a public road, so that by keeping in the middle one might cross unaware of the marvel. To realize its height it must be viewed from beneath; from the side of the creek it has a Gothic aspect; its immense walls, clad with forest-trees, its dizzy elevation, buttress-like masses, and aërial symmetry make this sublime arch one of those objects which impress the imagination with grace and grandeur all the more impressive because the mysterious work of Nature,—eloquent of the ages, and instinct with the latent forces of the universe. Equally remarkable, but in a diverse style, is the Giant's Causeway, whose innumerable black stone columns rise from two to four hundred feet above the water's edge in the County of Antrim, on the north coast of Ireland. These basaltic pillars are for the most part pentagonal, whose five sides are closely united, not in one conglomerate mass, but, articulated so aptly that to be traced the ball and socket must be disjointed.

The effect of statuary upon bridges is memorable: the Imperial statues which line that of Berlin form an impressive array; and whoever has seen the figures on the Bridge of Sant' Angelo at Home, when illuminated on a Carnival night, or the statues upon Santa Trinità at Florence, bathed in moonlight, and their outline distinctly revealed against sky and water, cannot but realize how harmoniously sculpture may illustrate and heighten the architecture of the bridge. More quaint than appropriate is pictorial embellishment; a beautiful Madonna or local saint placed midway or at either end of a bridge, especially one of mediæval form and fashion, seems appropriate; but elaborate painting, such as one sees at Lucerne, strikes us as more curious than desirable. The bridge which divides the town and crosses the Reuss is covered, yet most of the pictures are weather-stained; as no vehicles are allowed, foot-passengers can examine them at ease. They are in triangular frames, ten feet apart, but few have any technical merit. One series illustrates Swiss history; and the Kapellbrücke has the pictorial life of the Saint of the town; while the Mile Bridge exhibits a quaint and rough copy of the famous "Dance of Death."

In Switzerland what fearful ravines and foaming cascades do bridges cross! sometimes so aërial, and overhanging such precipices, as to justify to the imagination the name superstitiously bestowed on more than one, of the Devil's Bridge; while from few is a more lovely effect of near water seen than the "arrowy Rhone," as we gaze down upon its "blue rushing" beneath the bridge at Geneva. Perhaps the varied pictorial effects of bridges, at least in a city, are nowhere more striking than at Venice, whose five hundred, with their mellow tint and association with palatial architecture and streets of water, especially when revealed by the soft and radiant hues of an Italian sunset, present outlines, shapes, colors, and contrasts so harmonious and beautiful as to warm and haunt the imagination while they charm the eye. It is remarkable, as an artistic fact, how graciously these structures adapt themselves to such diverse scenes,—equally, though variously, picturesque amid the sturdy foliage and wild gorges of the Alps, the bustle, fog, and mast-forest of the Thames, and the crystal atmosphere, Byzantine edifices, and silent canals of Venice.

Whoever has truly felt the aërial perspective of Turner has attained a delicate sense of the pictorial significance of the bridge; for, as we look through his floating mists, we descry, amid Nature's most evanescent phenomena, the span, the arch, the connecting lines or masses whereby this familiar image seems to identify itself not less with Nature than with Art. Among the drawings which Arctic voyagers have brought home, many a bridge of ice, enormous and symmetrical, seems to tempt adventurous feet and to reflect a like form of fleecy cloud-land; daguerreotyped by the frost in miniature, the same structures may be traced on the window-pane; printed on the fossil and the strata of rock, in the veins of bark and the lips of shells, or floating in sunbeams, an identical design appears; and, on a summer morning, as the eye carefully roams over a lawn, how often do the most perfect little suspension-bridges hang from spear to spear of herbage, their filmy span embossed with glittering dew-drops!

INTERNAL STRUCTURE AND PROGRESSION OF THE GLACIER

It is not my intention, in these articles, to discuss a general theory of the glaciers upon physical and mechanical principles. My special studies, always limited to Natural History, have but indifferently fitted me for such a task, and quite recently the subject has been admirably treated from this point of view by Dr. Tyndall, in his charming volume entitled "Glaciers of the Alps." I have worked upon the glaciers as an amateur, devoting my summer vacations, with friends desirous of sharing my leisure, to excursions in the Alps, for the sake of relaxation from the closer application of my professional studies, and have considered them especially in their connection with geological phenomena, with a view of obtaining, by means of a thorough acquaintance with glaciers as they exist now, some insight into the glacial phenomena of past times, the distribution of drift, the transportation of boulders, etc. It was, however, impossible to treat one series of facts without some reference to the other; but such explanations as I have given of the mechanism of the glacier, in connection with its structure, are presented in the language of the unprofessional observer, without any attempt at the technicalities of the physicist. I do not wonder, therefore, that those who have looked upon the glacier chiefly with reference to the physical and mechanical principles involved in its structure and movement should have found my Natural Philosophy defective. I am satisfied with their agreement as to my correct observation of the facts, and am the less inclined to quarrel with the doubts thrown on my theory since I see that the most eminent physicists of the day do not differ from me more sharply than they do from each other. The facts will eventually test all our theories, and they form, after all, the only impartial jury to which we can appeal. In the mean while, I am not sorry that just at this moment, when recent investigations and publications have aroused new interest in the glaciers, the course of these articles brings me naturally to a discussion of the subject in its bearing upon geological questions. I shall, however, address myself especially, as I have done throughout these papers, to my unprofessional readers, who, while they admire the glaciers, may also wish to form a general idea of their structure and mode of action, as well as to know something of the important part they have played in the later geological history of our earth. It would, indeed, be out of place, were I to undertake here a discussion of the different views entertained by the various students who have investigated the glacier itself, among whom Dr. Tyndall is especially distinguished, or those of the more theoretical writers, among whom Mr. Hopkins occupies a prominent position.

Removed, as I am, from all possibility of renewing my own observations, begun in 1836 and ended in 1845, I will take this opportunity to call the attention of those particularly interested in the matter to one essential point with reference to which all other observers differ from me. I mean the stratification of the glacier, which I do not believe to be rightly understood, even at this moment. It may seem presumptuous to dissent absolutely from the statements of one who has seen so much and so well as Dr. Tyndall, on a question for the solution of which, from the physicist's point of view, his special studies have been a far better preparation than mine; and yet I feel confident that I was correct in describing the stratification of the glacier as a fundamental feature of its structure, and the so-called dirt-bands as the margins of the snow-strata successively deposited, and in no way originating in the ice-cascades. I shall endeavor to make this plain to my readers in the course of the present article. I believe, also, that renewed observations will satisfy dissenting observers that there really exists a net-work of capillary fissures extending throughout the whole glacier, constantly closing and reopening, and constituting the channels by means of which water filtrates into its mass. This infiltration, also, has been denied, in consequence of the failure of some experiments in which an attempt was made to introduce colored fluids into the glacier. To this I can only answer, that I succeeded completely, myself, in the self-same experiments which a later investigator found impracticable, and that I see no reason why the failure of the latter attempt should cast a doubt upon the former. The explanation of the difference in the result may, perhaps, be found in the fact, that, as a sponge gorged with water can admit no more fluid than it already contains, so the glacier, under certain circumstances, and especially at noonday in summer, may be so soaked with water that all attempts to pour colored fluids into it would necessarily fail. I have stated, in my work upon glaciers, that my infiltration-experiments were chiefly made at night; and I chose that time, because I knew the glacier would most readily admit an additional supply of liquid from without when the water formed during the day at its surface and rushing over it in myriad rills had ceased to flow.

While we admit a number of causes as affecting the motion of a glacier, namely, the natural tendency of heavy bodies to slide down a sloping surface, the pressure to which the mass is subjected forcing it onward, the infiltration of moisture, its freezing and consequent expansion,—we must also remember that these various causes, by which the accumulated masses of snow and ice are brought down from higher to lower levels, are not all acting at all times with the same intensity, nor is their action always the same at every point of the moving mass. While the bulk of snow and ice moves from higher to lower levels, the whole mass of the snow, in consequence of its own downward tendency, is also under a strong vertical pressure, arising from its own incumbent weight, and that pressure is, of course, greater at its bottom than at its centre or surface. It is therefore plain, that, inasmuch as the snow can be compressed by its own weight, it will be more compact at the bottom of such an accumulation than at its surface, this cause acting most powerfully at the upper part of a glacier, where the snow has not yet been transformed into a more solid icy mass. To these two agencies, the downward tendency and the vertical pressure, must be added the pressure from behind, which is most-effective where the mass is largest and the amount of motion in a given time greatest. In the glacier, the mass is, of course, largest in the centre, where the trough which holds it is deepest, and least on the margins, where the trough slopes upward and becomes more shallow. Consequently, the middle of a glacier always advances more rapidly than the sides. Were the slope of the ground over which it passes, combined with the pressure to which the mass is subjected, the whole secret of the onward progress of a glacier, it is evident that the rate of advance would be gradually accelerated, reaching its maximum at its lower extremity, and losing its impetus by degrees on the higher levels nearer the point where the descent begins. This, however, is not the case. The glacier of the Aar, for instance, is about ten miles in length; its rate of annual motion is greatest near the point of junction of the two great branches by which it is formed, diminishing farther down, and reaching a minimum at its lower extremity. But in the upper regions, near their origin, the progress of these branches is again gradually less. Let us see whether the next cause of displacement, the infiltration of moisture, may not in some measure explain this retardation, at least of the lower part of the glacier. This agency, like that of the compression of the snow by its own weight and the pressure from behind, is most effective where the accumulation is largest. In the centre, where the body of the mass is greatest, it will imbibe the most moisture. But here a modifying influence comes in, not sufficiently considered by the investigators of glacial structure. We have already seen that snow and ice at different degrees of compactness are not equally permeable to moisture. Above the line at which the annual winter snow melts, there is, of course, little moisture; but below that point, as soon as the temperature rises in summer sufficiently to melt the surface, the water easily penetrates the mass, passing through it more readily where the snow is lightest and least compact,—in short, where it has not begun its transformation into ice. A summer's day sends countless rills of water trickling through such a mass of snow. If the snow be loose and porous throughout, the water will pass through its whole thickness, accumulating at the bottom, so that the lower portion of the mass will be damper, more completely soaked with water, than the upper part; if, on the contrary, in consequence of the process previously described, alternate melting and freezing combined with pressure, the mass has assumed the character of icy snow, it does not admit moisture so readily, and still farther down, where the snow is actually transformed into pure compact ice, the amount of surface-water admitted into its structure will, of course, be greatly diminished. There may, however, be conditions under which even the looser snow is comparatively impervious to water; as, for instance, when rain falls upon a snow-field which has been long under a low temperature, and an ice-crust is formed upon its surface, preventing the water from penetrating below. Admitting, as I believe we must, that the water thus introduced into the snow and ice is one of the most powerful agents to which its motion is due, we must suppose that it has a twofold influence, since its action when fluid and when frozen would be different. When fluid, it would contribute to the advance of the mass in proportion to its quantity; but when frozen, its expansion would produce a displacement corresponding to the greater volume of ice as compared with water; add to this that while trickling through the mass it will loosen and displace the particles of already consolidated ice. I have already said that I did not intend to trespass on the ground of the physicist, and I will not enter here upon any discussion as to the probable action of the laws of hydrostatic pressure and dilatation in this connection. I will only state, that, so far as my own observation goes, the movement of the glacier is most rapid where the greatest amount of moisture is introduced into the mass, and that I believe there must be a direct relation between these two facts. If I am right in this, then the motion, so far as it is connected with infiltrated moisture or with the dilatation caused by the freezing of that moisture, will, of course, be most rapid where the glacier is most easily penetrated by water, namely, in the region of the névé and in the upper portion of the glacier-troughs, where the névé begins to be transformed into more or less porous ice. This cause also accounts, in part at least, for another singular fact in the motion of the glacier: that, in its higher levels, where its character is more porous and the water entering at the surface sinks readily to the bottom, there the bottom seems to move more rapidly than the superficial parts of the mass, whereas at the lower end of the glacier, in the region of the compact ice, where the infiltration of the water at the bottom is at its minimum, while the disintegrating influences at the surface admit of infiltration to a certain limited depth, there the motion is greater near the surface than toward the bottom. But, under all circumstances, it is plain that the various causes producing motion, gravitation, pressure, infiltration of water, frost, will combine to propel the mass at a greater rate along its axis than near its margins. For details concerning the facts of the case, I would refer to my work entitled "Système Glaciaire."

We will next consider the stratification of the glacier. I have stated in my introductory remarks, that I consider this to be one of its primary and fundamental features, and I confess, that, after a careful examination of the results obtained by my successors in the field of glacial phenomena, I still believe that the original stratification of the mass of snow from which the glacier arises gives us the key to many facts of its internal structure. The ultimate features resulting from this connection are so exceedingly intricate and entangled that their relation is not easily explained. Nevertheless, I trust my readers will follow me in this Alpine excursion, where I shall try to smooth the asperities of the road for them as much as possible.

Imparted to it, at the very beginning of its formation, by the manner in which snow accumulates, and retained through all its transformations, the stratification of a glacier, however distorted, and at times almost obliterated, remains, notwithstanding, as distinct to one who is acquainted with all its phases, as is the stratified character of metamorphic rocks to the skilful geologist, even though they may be readily mistaken for plutonic masses by the common observer. Indeed, even those secondary features, as the dirt-bands, for instance, which we shall see to be intimately connected with snow-strata, and which eventually become so prominent as to be mistaken for the cause of the lines of stratification, do nevertheless tend, when properly understood, to make the evidence of stratification more permanent, and to point out its primitive lines.

On the plains, in our latitude, we rarely have the accumulated layers of several successive snow-storms preserved one above another. We can, therefore, hardly imagine with what distinctness the sequence of such beds is marked in the upper Alpine regions. The first cause of this distinction between the layers is the quality of the snow when it falls, then the immediate changes it undergoes after its deposit, then the falling of mist or rain upon it, and lastly and most efficient of all, the accumulation of dust upon its surface. One who has not felt the violence of a storm in the high mountains, and seen the clouds of dust and sand carried along with the gusts of wind passing over a mountain-ridge and sweeping through the valley beyond, can hardly conceive that not only the superficial aspect of a glacier, but its internal structure also, can be materially affected by such a cause. Not only are dust and sand thus transported in large quantities to the higher mountain-regions, but leaves are frequently found strewn upon the upper glacier, and even pine-cones, and maple-seeds flying upward on their spread wings, are scattered thousands of feet above and many miles beyond the forests where they grew.

This accumulation of sand and dust goes on all the year round, but the amount accumulated over one and the same surface is greatest during the summer, when the largest expanse of rocky wall is bare of snow and its loose soil dried by the heat so as to be easily dislodged. This summer deposit of loose inorganic materials, light enough to be transported by the wind, forms the main line of division between the snow of one year and the next, though only that of the last year is visible for its whole extent. Those of the preceding years, as we shall see hereafter, exhibit only their edges cropping out lower down one beyond another, being brought successively to lower levels by the onward motion of the glacier.

Other observers of the glacier, Professor Forbes and Dr. Tyndall, have noticed only the edges of these seams, and called them dirt-bands. Looking upon them as merely superficial phenomena, they have given explanations of their appearance which I hold to be quite untenable. Indeed, to consider these successive lines of dirt on the glacier as limited only to its surface, and to explain them from that point of view, is much as if a geologist were to consider the lines presented by the strata on a cut through a sedimentary mass of rock as representing their whole extent, and to explain them as a superficial deposit due to external causes.

A few more details may help to make this statement clearer to my readers. Let us imagine that a fresh layer of snow has fallen in these mountain-regions, and that a deposit of dirt has been scattered over its surface, which, if any moisture arises from the melting of the snow or from the falling of rain or mist, will become more closely compacted with it. The next snow-storm deposits a fresh bed of snow, separated from the one below it by the sheet of dust just described, and this bed may, in its turn, receive a like deposit. For greater ease and simplicity of explanation, I speak here as if each successive snow-layer were thus indicated; of course this is not literally true, because snow-storms in the winter may follow each other so fast that there is no time for such a collection of foreign materials upon each newly formed surface. But whenever such a fresh snow-bed, or accumulation of beds, remains with its surface exposed for some time, such a deposit of dirt will inevitably be found upon it. This process may go on till we have a number of successive snow-layers divided from each other by thin sheets of dust. Of course, such seams, marking the stratification of snow, are as permanent and indelible as the seams of coarser materials alternating with the finest mud in a sedimentary rock.

The gradual progress of a glacier, which, though more rapid in summer than in winter, is never intermitted, must, of course, change the relation of these beds to each other. Their lower edge is annually cut off at a certain level, because the snow deposited every winter melts with the coming summer, up to a certain line, determined by the local climate of the place. But although the snow does not melt above this line, we have seen, in the preceding article, that it is prevented from accumulating indefinitely in the higher regions by its own tendency to move down to the lower valleys, and crowding itself between their walls, thus to force its way toward the outlet below. Now, as this movement is very gradual, it is evident that there must be a perceptible difference in the progress of the successive layers, the lower and older ones getting the advance of the upper and more recent ones: that is, when the snow that has covered the face of the country during one winter melts away from the glacier up to the so-called snow-line, there will be seen cropping out below and beyond that line the layers of the preceding years, which are already partially transformed into ice, and have become a part of the frozen mass of the glacier with which they are moving onward and downward. In the autumn, when the dust of a whole season has been accumulated upon the surface of the preceding winter's snow, the extent of the layer which year after year will henceforth crop out lower down, as a dirt-band, may best be appreciated.

Beside the snow-layers and the sheets of dust alternating with them, there is still another feature of the horizontal and parallel structure of the mass in immediate connection with those above considered. I allude to the layers of pure compact ice occurring at different intervals between the snow-layers. In July, when the snow of the preceding winter melts up to the line of perpetual snow, the masses above, which are to withstand the summer heat and become part of the glacier forever, or at least until they melt away at the lower end, begin to undergo the changes through which all snow passes before it acquires the character of glacial ice. It thaws at the surface, is rained upon, or condenses moisture, thus becoming gradually soaked, and after assuming the granular character of névé-ice, it ends in being transformed into pure compact ice. Toward the end of August, or early in September, when the nights are already very cold in the Alps, but prior to the first permanent autumnal snow-falls, the surface of these masses becomes frozen to a greater or less depth, varying, of course, according to temperature. These layers of ice become numerous and are parallel to each other, like the layers of ice formed from slosh. Such crusts of ice I have myself observed again and again upon the glacier. This stratified snowy ice is now the bottom on which the first autumnal snow-falls accumulate. These sheets of ice may be formed not only annually before the winter snows set in, but may recur at intervals whenever water accumulating upon an extensive snow-surface, either in consequence of melting or of rain, is frozen under a sharp frost before another deposit of snow takes place. Or suppose a fresh layer of light porous snow to have accumulated above one the surface of which has already been slightly glazed with frost; rain or dew, falling upon the upper one, will easily penetrate it; but when it reaches the lower one, it will be stopped by the film of ice already formed, and under a sufficiently low temperature, it will be frozen between the two. This result may be frequently noticed in winter, on the plains, where sudden changes of temperature take place.

There is still a third cause, to which the same result may possibly be due, and to which I shall refer at greater length hereafter; but as it has not, like the preceding ones, been the subject of direct observation, it must be considered as hypothetical. The admirable experiments of Dr. Tyndall have shown that water may be generated in ice by pressure, and it is therefore possible that at a lower depth in the glacier, where the incumbent weight of the mass above is sufficient to produce water, the water thus accumulated may be frozen into ice-layers. But this depends so much upon the internal temperature of the glacier, about which we know little beyond a comparatively superficial depth, that it cannot at present afford a sound basis even for conjecture.

There are, then, in the upper snow-fields three kinds of horizontal deposits: the beds of snow, the sheets of dust, and the layers of ice, alternating with each other. If, now, there were no modifying circumstances to change the outline and surface of the glacier,—if it moved on uninterruptedly through an open valley, the lower layers, forming the mass, getting by degrees the advance of the upper ones, our problem would be simple enough. We should then have a longitudinal mass of snow, inclosed between rocky walls, its surface crossed by straight transverse lines marking the annual additions to the glacier, as in the adjoining figure.

But that mass of snow, before it reaches the outlet of the valley, is to be compressed, contorted, folded, rent in a thousand directions. The beds of snow, which in the upper ranges of the mountain were spread out over broad, open surfaces, are to be crowded into comparatively circumscribed valleys, to force and press themselves through narrow passes, alternately melting and freezing, till they pass from the condition of snow into that of ice, to undergo, in short, constant transformations, by which the primitive stratification will be extensively modified. In the first place, the more rapid motion of the centre of the glacier, as compared with the margins, will draw the lines of stratification downward toward the middle faster than at the sides. Accurate measurements have shown that the axis of a glacier may move ten- or twenty-fold more rapidly than its margins. This is not the place to introduce a detailed account of the experiments made to ascertain this result; but I would refer those who are interested in the matter to the measurements given in my "Système Glaciaire," where it will be seen that the middle may move at a rate of two hundred feet a year, while the margins may not advance more than ten or fifteen or twenty feet. These observations of mine have the advantage over those of other observers, that, while they embrace the whole extent of the glacier, transversely as well as in its length, they cover a period of several successive years, instead of being limited to summer campaigns and a few winter observations. The consequence of this mode of progressing will be that the straight lines drawn transversely across the surface of the glacier above will be gradually changed to curved ones below. After a few years, such a line will appear on the surface of the glacier like a crescent, with the bow turned downward, within which, above, are other crescents, less and less sharply arched up to the last year's line, which may be again straight across the snow-field. (See the subjoined figure, which represents a part of the glacier of the Lauter-Aar.)

Thus the glacier records upon its surface its annual growth and progress, and registers also the inequality in the rate of advance between the axis and the sides.

But these are only surface-phenomena. Let us see what will be the effect upon the internal structure. We must not forget, in considering the changes taking place within glaciers, the shape of the valleys which contain them. A glacier lies in a deep trough, and the tendency of the mass will be to sink toward its deeper part, and to fold inward and downward, if subjected to a strong lateral pressure,—that is, to dip toward the centre and slope upward along the sides, following the scoop of the trough. If, now, we examine the face of a transverse cut in the glacier, we find it traversed by a number of lines, vertical in some places, more or less oblique in others, and frequently these lines are joined together at the lower ends, forming loops, some of which are close and vertical, while others are quite open. These lines are due to the folding of the strata in consequence of the lateral pressure they are subjected to, when crowded into the lower course of the valleys, and the difference in their dip is due to the greater or less force of that pressure. The wood-cut on the next page represents a transverse cut across the Lauter-Aar and the Finster-Aar, the two principal tributaries to the great Aar glacier, and includes also a number of small lateral glaciers which join them. The beds on the left, which dip least, and are only folded gently downward, forming very open loops, are those of the Lauter-Aar, where the lateral pressure is comparatively slight. Those which are almost vertical belong in part to the several small tributary glaciers, which have been crowded together and very strongly compressed, and partly to the Finster-Aar. The close uniform vertical lines in this wood-cut represent a different feature in the structure of the glacier, called blue bands, to which I shall refer presently. These loops or lines dipping into the internal mass of the glacier have been the subject of much discussion, and various theories have been recently proposed respecting them. I believe them to be caused, as I have said, by the snow-layers, originally deposited horizontally, but afterward folded into a more or less vertical position, in consequence of the lateral pressure brought to bear upon them. The sheets of dust and of ice alternating with the snow-strata are of course subjected to the same action, and are contorted, bent, and folded by the same lateral pressure.

Dr. Tyndall has advanced the view that the lines of apparent stratification, and especially the dirt-bands across the surface of the glacier, are due to ice-cascades: that is, the glacier, passing over a sharp angle, is cracked across transversely in consequence of the tension, and these rents, where the back of the glacier has been successively broken, when recompacted, cause the transverse lines, the dirt being collected in the furrow formed between the successive ridges. Unfortunately for his theory, the lines of stratification constantly occur in glaciers where no such ice-falls are found. His principal observations upon this subject were made on the Glacier du Géant, where the ice-cascade is very remarkable. The lines may perhaps be rendered more distinct on the Glacier du Géant by the cascade, and necessarily must be so, if the rents coincide with the limit at which the annual snow-line is nearly straight across the glacier. In the region of the Aar glacier, however, where my own investigations were made, all the tributaries entering into the larger glacier are ribbed across in this way, and most of them join the main trunk over uniform slopes, without the slightest cascade.

It must be remembered that these surface-phenomena of the glacier are not to be seen at all times, nor under all conditions. During the first year of my sojourn on the glacier of the Aar, I was not aware that the stratification of its tributaries was so universal as I afterward found it to be; the primitive lines of the strata are often so far erased that they are not perceptible, except under the most favorable circumstances. But when the glacier has been washed clean by rain, and the light strikes upon it in the right direction, these lines become perfectly distinct, where, under different conditions, they could not be discerned at all. After passing many summers on the same glacier, renewing my observations year after year over the same localities, I can confidently state that not only do the lines of stratification exist throughout the great glacier of the Aar, but in all its tributaries also. Of course, they are greatly modified in the lower part of the glacier by the intimate fusion of its tributaries, and by the circumstance that their movement, primarily independent, is merged in the movement of the main glacier embracing them all. We have seen that not only does the centre of a glacier move more rapidly than its sides, but that the deeper mass of the glacier also moves at a different rate from its more superficial portion. My own observations (for the details of which I would again refer the reader to my "Système Glaciaire ") show that in the higher part of the glacier, especially in the region of the névé, the bottom of the mass seems to move more rapidly than the surface, while lower down, toward the terminus of the glacier, the surface, on the contrary, moves faster than the bottom. The annexed wood-cut exhibits a longitudinal section of the glacier, in which this difference in the motion of the upper and lower portions of the mass is represented, the beds being almost horizontal in the upper snow-fields, while their lower portion slopes move rapidly downward in the névé region, and toward the lower end the upper portion takes the lead, and advances more rapidly than the lower.

I presented these results for the first time in two letters, dated October 9th, 1842, which were published in a German periodical, the Jahrbuch of Leonhard and Bronn. The last three wood-cuts introduced above, the transverse and longitudinal sections of the glacier as well as that representing the concentric lines of stratification on the surface, are the identical ones contained in those communications. These papers seem to have been overlooked by contemporary investigators, and I may be permitted to translate here a passage from one of them, since it sums up the results of the inequality of motion throughout the glacier and its influence on the primitive stratification of the mass in as few words and as correctly as I could give them to-day, twenty years later:—"Combining these views, it appears that the glacier may be represented as composed of concentric shells which arise from the parallel strata of the upper region by the following process. The primitively regular strata advance into gradually narrower and deeper valleys, in consequence of which the margins are raised, while the middle is bent not only downward, but, from its more rapid motion, forward also, so that they assume a trough-like form in the interior of the mass. Lower down, the glacier is worn by the surrounding air, and assumes the peculiar form characteristic of its lower course." The last clause alludes to another series of facts, which we shall examine in a future article, when we shall see that the heat of the walls in the lower part of its course melts the sides of the glacier, so that, instead of following the trough-like shape of the valley, it becomes convex, arching upward in the centre and sinking at the margins.

I have dwelt thus long, and perhaps my readers may think tediously, upon this part of my subject, because the stratification of the glacier has been constantly questioned by the more recent investigators of glacial phenomena, and has indeed been set aside as an exploded theory. They consider the lines of stratification, the dirt-bands, and the seams of ice alternating with the more porous snow, as disconnected surface-phenomena, while I believe them all to be intimately connected together as primary essential features of the original mass.

There is another feature of glacial structure, intimately connected, by similarity of position and aspect, with the stratification, which has greatly perplexed the students of glacial phenomena. I allude to the so-called blue bands, or bands of infiltration, also designated as veined structure, ribboned or laminated structure, marginal structure, and longitudinal structure. The difficulty lies, I believe, in the fact that two very distinct structures, that of the stratification and the blue bands, are frequently blended together in certain parts of the glacier in such a manner as to seem identical, while elsewhere the one is prominent and the other subordinate, and vice versâ. According to their various opportunities of investigation, observers have either confounded the two, believing them to be the same, or some have overlooked the one and insisted upon the other as the prevailing feature, while that very feature has been absolutely denied again by others who have seen its fellow only, and taken that to be the only prominent and important fact in this peculiar structural character of the ice.

We have already seen how the stratification of the glacier arises, accompanied by layers of dust and other material foreign to the glacier, and how blue bands of compact ice may be formed parallel to the surface of these strata. We have also seen how the horizontality of these strata may be modified by pressure till they assume a position within the mass of the glacier, varying from a slightly oblique inclination to a vertical one. Now, while the position of the strata becomes thus altered under pressure, other changes take place in the constitution of the ice itself.

Before attempting to explain how these changes take place, let us consider the facts themselves. The mass of the glacial ice is traversed by thin bands of compact blue ice, these bands being very numerous along the margins of the glacier, where they constitute what Dr. Tyndall calls marginal structure, and still more crowded along the line upon which two glaciers unite, where he has called it longitudinal structure. In the latter case, where the extreme pressure resulting from the junction of two glaciers has rendered the strata nearly vertical, these blue bands follow their trend so closely that it is difficult to distinguish one from the other. It will be seen, on referring to the wood-cut on page 758, where the close, uniform, vertical lines represent the true veined structure, that at several points of that section the lines of stratification run so nearly parallel with them, that, were the former not drawn more strongly, they could not be easily distinguished from the latter. Along the margins, also, in consequence of the retarded motion, the blue bands and the lines of stratification run nearly parallel with each other, both following the sides of the trough in which they move.

Undoubtedly, in both these instances, we have two kinds of blue bands, namely: those formed primitively in a horizontal position, indicating seams of stratification, and those which have arisen subsequently in connection with the movement of the whole mass, which I have occasionally called bands of infiltration, as they appeared to me to be formed by the infiltration and freezing of water. The fact that these blue bands are most numerous where two glaciers are crowded together into a common bed naturally suggests pressure as their cause. And since the beautiful experiments of Dr. Tyndall have illustrated the internal liquefaction of ice by pressure, it becomes highly probable that his theory of the origin of these secondary blue bands is the true one. He suggests that layers of water may be formed in the glacier at right angles with the pressure, and pass into a state of solid ice upon the removal of that pressure, the pressure being of course relieved in proportion to the diminution in the body of the ice by compression. The number of blue bands diminishes as we recede from the source of the pressure,—few only being formed, usually at right angles with the surfaces of stratification, in the middle of a glacier, half-way between its sides. If they are caused by pressure, this diminution of their number toward the middle of the glacier would be inevitable, since the intensity of the pressure naturally fades as we recede from the motive power.

Dr. Tyndall also alludes to another structure of the same kind, which he calls transverse structure, where the blue bands extend in crescent-shaped curves, more or less arched, across the surface of the glacier. Where these do not coincide with the stratification, they are probably formed by vertical pressure in connection with the unequal movement of the mass.

With these facts before us, it seems to me plain that the primitive blue bands arise with the stratification of the snow in the very first formation of the glacier, while the secondary blue bands are formed subsequently, in consequence of the onward progress of the glacier and the pressure to which it is subjected. The secondary blue bands intersect the planes of stratification at every possible angle, and may therefore seem identical with the stratification in some places, while in others they cut it at right angles. It has been objected to my theory of glacial structure, that I have considered the so-called blue bands as a superficial feature when compared with the stratification. And in a certain sense this is true; since, if my views are correct, the glacier exists and is in full life and activity before the secondary blue bands arise in it, whereas the stratification is a feature of its embryo condition, already established in the accumulated snow before it begins its transformation into glacier-ice. In other words, the veined structure of the glacier is not a primary structural feature of its whole mass, but the result of various local influences acting upon the constitution of the ice: the marginal structure resulting from the resistance of the sides of the valley to the onward movement of the glacier, the longitudinal structure arising from the pressure caused by two glaciers uniting in one common bed, the transverse structure being produced by vertical pressure in consequence of the weight of the mass itself and the increased rate of motion at the centre.

In the névé fields, where the strata are still horizontal, the few blue bands observed are perpendicular to the strata of snow, and therefore also perpendicular to the blue seams of ice and the sheets of dust alternating with them. Upon the sides of the glacier they are more or less parallel to the slopes of the valley; along the line of junction of two glaciers they follow the vertical trend of the axis of the mass; while at intermediate positions they are more or less oblique. Along the outcropping edges of the strata, on the surface of the glacier, they follow more or less the dip of the strata themselves; that is to say, they are more or less parallel with the dirt-bands. In conclusion, I would recommend future investigators to examine the glaciers, with reference to the distribution of the blue bands, after heavy rains and during foggy days, when the surface is freed from the loose materials and decomposed fragments of ice resulting from the prolonged action of the sun.

The most important facts, then, to be considered with reference to the motion of the glacier are as follows. First that the rate of advance between the axis and the margins of a glacier differs in the ratio of about ten to one and even less; that is to say; when the centre is advancing at a rate of two hundred and fifty feet a year, the motion toward the sides may be gradually diminished to two hundred, one hundred and fifty, one hundred, fifty feet, and so on, till nearest the margin it becomes almost inappreciable. Secondly, the rate of motion is not the same throughout the length of the glacier, the advance being greatest about half-way down in the region of the névé, and diminishing in rapidity both above and below; thus the onward motion in the higher portion of a glacier may not exceed twenty to fifty feet a year, while it reaches its maximum of some two hundred and fifty feet annually in the névé region, and is retarded again toward the lower extremity, where it is reduced to about one-fourth of its maximum rate. Thirdly, the glacier moves at different rates throughout the thickness of its mass; toward the lower extremity of the glacier the bottom is retarded, and the surface portion moves faster, while in the upper region the bottom seems to advance more rapidly. I say seems, because upon this latter point there are no positive measurements, and it is only inferred from general appearances, while the former statement has been demonstrated by accurate experiments. Remembering the form of the troughs in which the glaciers arise, that they have their source in expansive, open fields of snow and névé, and that these immense accumulations move gradually down into ever narrowing channels, though at times widening again to contract anew, their surface wasting so little from external influences that they advance far below the line of perpetual snow without any sensible diminution in size, it is evident that an enormous pressure must have been brought to bear upon them before they could have been packed into the lower valleys through which they descend.

Physicists seem now to agree that pressure is the chief agency in the motion of glaciers. No doubt, all the facts point that way; but it now becomes a matter of philosophical interest to determine in what direction it acts most powerfully, and upon this point glacialists are by no means agreed. The latest conclusion seems to be, that the weight of the advancing mass is itself the efficient cause of the motion. But while this is probably true in the main, other elements tending to the same result, and generally overlooked by investigators, ought to be taken into consideration; and before leaving the subject, I would add a few words upon infiltration in this connection.

The weight of the glacier, as a whole, is about the same all the year round. If, therefore, pressure, resulting from that weight, be the all-controlling agency, its progress should be uniform daring the whole year, or even greatest in winter, which is by no means the case. By a series of experiments, I have ascertained that the onward movement, whatever be its annual average, is accelerated in spring and early summer. The average annual advance of the glacier being, at a given point, at the rate of about two hundred feet, its average summer advance, at the same point, will be at a rate of two hundred and fifty feet, while its average rate of movement in winter will be about one hundred and fifty feet. This can be accounted for only by the increased pressure due to the large accession of water trickling in spring and early summer into the interior through the net-work of capillary fissures pervading the whole mass. The unusually large infiltration of water at that season is owing to the melting of the winter snow. Careful experiments made on the glacier of the Aar, respecting the water thus accumulating on the surface, penetrating its mass, and finally discharged in part at its lower extremity, fully confirm this view. Here, then, is a powerful cause of pressure and consequent motion, quite distinct from the permanent weight of the mass itself, since it operates only at certain seasons of the year. In midwinter, when the infiltration is reduced to a minimum, the motion is least. The water thus introduced into the glacier acts, as we have seen above, in various ways: by its weight, by loosening the particles of snow through which it trickles, and by freezing and consequent expansion, at least within the limits and during the season at which the temperature of the glacier sinks below 32° Fahrenheit. The simple fact, that in the spring the glacier swells on an average to about five feet more than its usual level, shows how important this infiltration must be. I can therefore only wonder that other glacialists have given so little weight to this fact. It is admitted by all, that the waste of a glacier at its surface, in consequence of evaporation and melting, amounts to about nine or ten feet in a year. At this rate of diminution, a glacier, even one thousand feet in thickness, could not advance during a single century without being exhausted. The water supplied by infiltration no doubt repairs the loss to a great degree. Indeed, the lower part of the glacier must be chiefly maintained from this source, since the annual increase from the fresh accumulations of snow is felt only above the snow-line, below which the yearly snow melts away and disappears. In a complete theory of the glaciers, the effect of so great an accession of plastic material cannot be overlooked.

I now come to some points in the structure of the glacier, the consideration of which is likely to have a decided influence in settling the conflicting views respecting their motion. The experiments of Faraday concerning regelation, and the application of the facts made known by the great English physicist to the theory of the glaciers, as first presented by Dr. Tyndall in his admirable work, show that fragments of ice with most surfaces are readily reunited under pressure into a solid mass. It follows from these experiments, that glacier-ice, at a temperature of 32° Fahrenheit, may change its form and preserve its continuity during its motion, in virtue of the pressure to which it is subjected. The statement is, that, when two pieces of ice with moistened surfaces are placed in contact, they become cemented together by the freezing of a film of water between them, while, when the ice is below 32° Fahrenheit, and therefore dry, no effect of the kind can be produced. The freezing was also found to take place under water; and the result was the same, even when the water into which the ice was plunged was as hot as the hand can bear.

The fact that ice becomes cemented under these circumstances is fully established, and my own experiments have confirmed it to the fullest extent. I question, however, the statement, that regelation takes place by the freezing of a film of water between the fragments. I never have been able to detect any indication of the presence of such a film, and am, therefore, inclined to consider this result as akin to what takes place when fragments of moist clay or marl are pressed together and thus reunited. When examining beds of clay and marl, or even of compact limestone, especially in large mountain-masses, I have frequently observed that the rock presents a net-work of minute fissures pervading the whole, without producing a distinct solution of continuity, though generally determining the lines according to which it breaks under sudden shocks. The net-work of capillary fissures pervading the glacier may fairly be compared to these rents in hard rocks,—with this difference, however, that in ice they are more permeable to water than in stone.

How this net-work of capillary fissures is formed has not been ascertained by direct observation. Following, however, the transformation of the snow and névé into compact ice, it is easily conceived that the porous mass of snow, as it falls in the upper regions of the Alps, and in the broad caldrons in which the glaciers properly originate, cannot pass into solid ice, by the process described in a former article, without retaining within itself larger or smaller quantities of air. This air is finally surrounded from all sides by the cementation of the granules of névé, through the freezing of the water that penetrates it. So inclosed, the bubbles of air are subject to the same compression as the ice itself, and become more flattened in proportion as the snow has been more fully transformed into compact ice. As long as the transformation of snow into ice is not complete, a rise of its temperature to 32° Fahrenheit, accompanied with thawing, reduces it at once again to the condition of loose grains of névé; but when more compact, it always presents the aspect of a mass composed of angular fragments, wedged and dove-tailed together, and separated by capillary fissures, the flattened air-bubbles trending in the same direction in each fragment, but varying in their trend from one fragment to another. There is, moreover, this important point to notice,—that, the older the névé, the larger are its composing granules; and where névé passes into porous ice, small angular fragments are mixed with rounded névé-granules, the angular fragments appearing larger and more numerous, and the névé-granules fewer, in proportion as the névé-ice has undergone most completely its transformation into compact glacier-ice. These facts show conclusively that the dimensions and form of the névé-granules, the size and shape of the angular fragments, the porosity of the ice, the arrangement of its capillary fissures, and the distribution and compression of the air-bubbles it contains, are all connected features, mutually dependent. Whether the transformation of snow into ice be the result of pressure only, or, as I believe, quite as much the result of successive thawings and freezings, these structural features can equally be produced, and exhibit these relations to one another. It may be, moreover, that, when the glacier is at a temperature below 32°, its motion produces extensive fissuration throughout the mass.