Geological Observations on South America

Comparative ranges of the extinct and living mollusca on the West Coast of

S. America.

Climate of the tertiary period.

On the causes of the absence of recent conchiferous deposits on the coast of S. America.

On the contemporaneous deposition and preservation of sedimentary formations.

RIO NEGRO.

I can add little to the details given by M. d'Orbigny on the sandstone formation of this district. ("Voyage" Part Geolog. pages 57-65.) The cliffs to the south of the river are about two hundred feet in height, and are composed of sandstone of various tints and degrees of hardness. One layer, which thinned out at both ends, consisted of earthy matter, of a pale reddish colour, with some gypsum, and very like (I speak after comparison of the specimens brought home) Pampean mud: above this was a layer of compact marly rock with dendritic manganese. Many blocks of a conglomerate of pumice-pebbles embedded in hard sandstone were strewed at the foot of the cliff, and had evidently fallen from above. A few miles N.E. of the town, I found, low down in the sandstone, a bed, a few inches in thickness, of a white, friable, harsh-feeling sediment, which adheres to the tongue, is of easy fusibility, and of little specific gravity; examined under the microscope, it is seen to be pumiceous tuff, formed of broken transparent crystals. In the cliffs south of the river, there is, also, a thin layer of nearly similar nature, but finer grained, and not so white; it might easily have been mistaken for a calcareous tuff, but it contains no lime: this substance precisely resembles a most widely extended and thick formation in Southern Patagonia, hereafter to be described, and which is remarkable for being partially formed of infusoria. These beds, conjointly with the conglomerate of pumice, are interesting, as showing the nature of the volcanic action in the Cordillera during this old tertiary period.

In a bed at the base of the southern cliffs, M. d'Orbigny found two extinct fresh-water shells, namely, a Unio and Chilina. This bed rested on one with bones of an extinct rodent, namely, the Megamys Patagoniensis; and this again on another with extinct marine shells. The species found by M. d'Orbigny in different parts of this formation consist of: —

1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole coast of Patagonia). 2. Ostrea Ferrarisi, d'Orbigny, "Voyage, Pal." 3. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe, and S. Josef). 4. Pecten Patagoniensis, d'Orbigny, "Voyage, Pal." 5. Venus Munsterii, d'Orbigny, "Voyage, Pal." (also at St. Fe). 6. Arca Bonplandiana, d'Orbigny, "Voyage, Pal." (also at St. Fe).

According to M. d'Orbigny, the sandstone extends westward along the coast as far as Port S. Antonio, and up the R. Negro far into the interior: northward I traced it to the southern side of the Rio Colorado, where it forms a low denuded plain. This formation, though contemporaneous with that of the rest of Patagonia, is quite different in mineralogical composition, being connected with it only by the one thin white layer: this difference may be reasonably attributed to the sediment brought down in ancient times by the Rio Negro; by which agency, also, we can understand the presence of the fresh-water shells, and of the bones of land animals. Judging from the identity of four of the above shells, this formation is contemporaneous (as remarked by M. d'Orbigny) with that under the Pampean deposit in Entre Rios and in Banda Oriental. The gravel capping the sandstone plain, with its calcareous cement and nodules of gypsum, is probably, from the reasons given in the First Chapter, contemporaneous with the uppermost beds of the Pampean formation on the upper plain north of the Colorado.

SAN JOSEF.

My examination here was very short: the cliffs are about a hundred feet high; the lower third consists of yellowish-brown, soft, slightly calcareous, muddy sandstone, parts of which when struck emit a fetid smell. In this bed the great Ostraea Patagonica, often marked with dendritic manganese and small coral-lines, were extraordinarily numerous. I found here the following shells: —

1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe and whole coast of Patagonia). 2. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe and R. Negro). 3. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Julian, and Port Desire). 4. Pecten Darwinianus, d'Orbigny, "Voyage, Pal." (also at St. Fe). 5. Pecten actinodes, G.B. Sowerby. 6. Terebratula Patagonica, G.B. Sowerby (also S. Julian). 7. Casts of a Turritella.

The four first of these species occur at St. Fe in Entre Rios, and the two first in the sandstone of the Rio Negro. Above this fossiliferous mass, there is a stratum of very fine-grained, pale brown mudstone, including numerous laminae of selenite. All the strata appear horizontal, but when followed by the eye for a long distance, they are seen to have a small easterly dip. On the surface we have the porphyritic gravel, and on it sand with recent shells.

NUEVO GULF.

From specimens and notes given me by Lieutenant Stokes, it appears that the lower bed consists of soft muddy sandstone, like that of S. Josef, with many imperfect shells, including the Pecten Paranensis, d'Orbigny, casts of a Turritella and Scutella. On this there are two strata of the pale brown mudstone, also like that of S. Josef, separated by a darker-coloured, more argillaceous variety, including the Ostrea Patagonica. Professor Ehrenberg has examined this mudstone for me: he finds in it three already known microscopic organisms, enveloped in a fine-grained pumiceous tuff, which I shall have immediately to describe in detail. Specimens brought to me from the uppermost bed, north of the Rio Chupat, consist of this same substance, but of a whiter colour.

Tertiary strata, such as here described, appear to extend along the whole coast between Rio Chupat and Port Desire, except where interrupted by the underlying claystone porphyry, and by some metamorphic rocks; these hard rocks, I may add, are found at intervals over a space of about five degrees of latitude, from Point Union to a point between Port S. Julian and S. Cruz, and will be described in the ensuing chapter. Many gigantic specimens of the Ostraea Patagonica were collected in the Gulf of St. George.

PORT DESIRE.

A good section of the lowest fossiliferous mass, about forty feet in thickness, resting on claystone porphyry, is exhibited a few miles south of the harbour. The shells sufficiently perfect to be recognised consist of: —

1. Ostrea Patagonica, d'Orbigny, (also at St. Fe, and whole coast of Patagonia). 2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Josef, S. Julian). 3. Pecten centralis, G.B. Sowerby (also at S. Julian and S. Cruz). 4. Cucullaea alta, G.B. Sowerby (also at S. Cruz). 5. Nucula ornata, G.B. Sowerby. 6. Turritella Patagonica, G.B. Sowerby.

The fossiliferous strata, when not denuded, are conformably covered by a considerable thickness of the fine-grained pumiceous mudstone, divided into two masses: the lower half is very fine-grained, slightly unctuous, and so compact as to break with a semi-conchoidal fracture, though yielding to the nail; it includes laminae of selenite: the upper half precisely resembles the one layer at the Rio Negro, and with the exception of being whiter, the upper beds at San Josef and Nuevo Gulf. In neither mass is there any trace to the naked eye of organic forms. Taking the entire deposit, it is generally quite white, or yellowish, or feebly tinted with green; it is either almost friable under the finger, or as hard as chalk; it is of easy fusibility, of little specific gravity, is not harsh to the touch, adheres to the tongue, and when breathed on exhales a strong aluminous odour; it sometimes contains a very little calcareous matter, and traces (besides the included laminae) of gypsum. Under the microscope, according to Professor Ehrenberg, it consists of minute, triturated, cellular, glassy fragments of pumice, with some broken crystals. ("Monatsberichten de konig. Akad. zu Berlin" vom April 1845.) In the minute glassy fragments, Professor Ehrenberg recognises organic structures, which have been affected by volcanic heat: in the specimens from this place, and from Port S. Julian, he finds sixteen Polygastrica and twelve Phytolitharia. Of these organisms, seven are new forms, the others being previously known: all are of marine, and chiefly of oceanic, origin. This deposit to the naked eye resembles the crust which often appears on weathered surfaces of feldspathic rocks; it likewise resembles those beds of earthy feldspathic matter, sometimes interstratified with porphyritic rocks, as is the case in this very district with the underlying purple claystone porphyry. From examining specimens under a common microscope, and comparing them with other specimens undoubtedly of volcanic origin, I had come to the same conclusion with Professor Ehrenberg, namely, that this great deposit, in its first origin, is of volcanic nature.

PORT S. JULIAN.

(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY FEET PLAIN AT PORT S. JULIAN.

(Section through beds from top to bottom: A, B, C, D, E, F.))

On the south side of the harbour, Figure 17 gives the nature of the beds seen in the cliffs of the ninety feet plain. Beginning at the top: —

1st, the earthy mass (AA), including the remains of the Macrauchenia, with recent shells on the surface.

Second, the porphyritic shingle (B), which in its lower part is interstratified (owing, I believe, to redisposition during denudation) with the white pumiceous mudstone.

Third, this white mudstone, about twenty feet in thickness, and divided into two varieties (C and D), both closely resembling the lower, fine- grained, more unctuous and compact kind at Port Desire; and, as at that place, including much selenite.

Fourth, a fossiliferous mass, divided into three main beds, of which the uppermost is thin, and consists of ferruginous sandstone, with many shells of the great oyster and Pecten Paranensis; the middle bed (E) is a yellowish earthy sandstone abounding with Scutellae; and the lowest bed (F) is an indurated, greenish, sandy clay, including large concretions of calcareous sandstone, many shells of the great oyster, and in parts almost made up of fragments of Balanidae. Out of these three beds, I procured the following twelve species, of which the two first were exceedingly numerous in individuals, as were the Terebratulae and Turritellae in certain layers: —

1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole coast of Patagonia). 2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B. Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B. Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6. Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9. Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella, differing from the species from Nuevo Gulf.

At the head of the inner harbour of Port S. Julian, the fossiliferous mass is not displayed, and the sea-cliffs from the water's edge to a height of between one and two hundred feet are formed of the white pumiceous mudstone, which here includes innumerable, far-extended, sometimes horizontal, sometimes inclined or vertical laminae of transparent gypsum, often about an inch in thickness. Further inland, with the exception of the superficial gravel, the whole thickness of the truncated hills, which represent a formerly continuous plain 950 feet in height, appears to be formed of this white mudstone: here and there, however, at various heights, thin earthy layers, containing the great oyster, Pecten Paranensis and Turritella ambulacrum, are interstratified; thus showing that the whole mass belongs to the same epoch. I nowhere found even a fragment of a shell actually in the white deposit, and only a single cast of a Turritella. Out of the eighteen microscopic organisms discovered by Ehrenberg in the specimens from this place, ten are common to the same deposit at Port Desire. I may add that specimens of this white mudstone, with the same identical characters were brought me from two points, – one twenty miles north of S. Julian, where a wide gravel-capped plain, 350 feet in height, is thus composed; and the other forty miles south of S. Julian, where, on the old charts, the cliffs are marked as "Chalk Hills."

SANTA CRUZ.

The gravel-capped cliffs at the mouth of the river are 355 feet in height: the lower part, to a thickness of fifty or sixty feet, consists of a more or less hardened, darkish, muddy, or argillaceous sandstone (like the lowest bed of Port Desire), containing very many shells, some silicified and some converted into yellow calcareous spar. The great oyster is here numerous in layers; the Trigonocelia and Turritella are also very numerous: it is remarkable that the Pecten Paranensis, so common in all other parts of the coast, is here absent: the shells consist of: —

1. Ostrea Patagonica, d'Orbigny; "Voyage Pal." (also at St. Fe and whole coast of Patagonia). 2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian). 3. Venus meridionalis of G.B. Sowerby. 4. Crassatella Lyellii, G.B. Sowerby. 5. Cardium puelchum, G.B. Sowerby. 6. Cardita Patagonica, G.B. Sowerby. 7. Mactra rugata, G.B. Sowerby. 8. Mactra Darwinii, G.B. Sowerby. 9. Cucullaea alta, G.B. Sowerby (also P. Desire). 10. Trigonocelia insolita, G.B. Sowerby. 11. Nucula (?) glabra, G.B. Sowerby. 12. Crepidula gregaria, G.B. Sowerby. 13. Voluta alta, G.B. Sowerby. 14. Trochus collaris, G.B. Sowerby. 15. Natica solida (?), G.B. Sowerby 16. Struthiolaria ornata, G.B. Sowerby (also P. Desire). 17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian). Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.

The upper part of the cliff is generally divided into three great strata, differing slightly in composition, but essentially resembling the pumiceous mudstone of the places farther north; the deposit, however, here is more arenaceous, of greater specific gravity, and not so white: it is interlaced with numerous thin veins, partially or quite filled with transverse fibres of gypsum; these fibres were too short to reach across the vein, have their extremities curved or bent: in the same veins with the gypsum, and likewise in separate veins as well as in little nests, there is much powdery sulphate of magnesia (as ascertained by Mr. Reeks) in an uncompressed form: I believe that this salt has not heretofore been found in veins. Of the three beds, the central one is the most compact, and more like ordinary sandstone: it includes numerous flattened spherical concretions, often united like a necklace, composed of hard calcareous sandstone, containing a few shells: some of these concretions were four feet in diameter, and in a horizontal line nine feet apart, showing that the calcareous matter must have been drawn to the centres of attraction, from a distance of four feet and a half on both sides. In the upper and lower finer-grained strata, there were other concretions of a grey colour, containing calcareous matter, and so fine-grained and compact, as almost to resemble porcelain- rock: I have seen exactly similar concretions in a volcanic tufaceous bed in Chiloe. Although in this upper fine-grained strata, organic remains were very rare, yet I noticed a few of the great oyster; and in one included soft ferruginous layer, there were some specimens of the Cucullaea alta (found at Port Desire in the lower fossiliferous mass) and of the Mactra rugata, which latter shell has been partially converted into gypsum.

(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S. CRUZ.

(Section through strata (from top to bottom)): Surface of plain with erratic boulders; 1,146 feet above the sea. a. Gravel and boulders, 212 feet thick. b. Basaltic lava, 322 feet thick. c, d and e. Sedimentary layers, bed of small pebbles and talus respectively, total 592 feet thick. River of S. Cruz; here 280 feet above sea.)

In ascending the valley of the S. Cruz, the upper strata of the coast- cliffs are prolonged, with nearly the same characters, for fifty miles: at about this point, they begin in the most gradual and scarcely perceptible manner, to be banded with white lines; and after ascending ten miles farther, we meet with distinct thin layers of whitish, greenish, and yellowish fine-grained, fusible sediments. At eighty miles from the coast, in a cliff thus composed, there were a few layers of ferruginous sandstone, and of an argillaceous sandstone with concretions of marl like those in the Pampas. (At this spot, for a space of three-quarters of a mile along the north side of the river, and for a width of half a mile, there has been a great slip, which has formed hills between sixty and seventy feet in height, and has tilted the strata into highly inclined and even vertical positions. The strata generally dipped at an angle of 45 degrees towards the cliff from which they had slided. I have observed in slips, both on a small and large scale, that this inward dip is very general. Is it due to the hydrostatic pressure of water percolating with difficulty through the strata acting with greater force at the base of the mass than against the upper part?) At one hundred miles from the coast, that is at a central point between the Atlantic and the Cordillera, we have the section in Figure 18.

The upper half of the sedimentary mass, under the basaltic lava, consists of innumerable zones of perfectly white bright green, yellowish and brownish, fine-grained, sometimes incoherent, sedimentary matter. The white, pumiceous, trachytic tuff-like varieties are of rather greater specific gravity than the pumiceous mudstone on the coast to the north; some of the layers, especially the browner ones, are coarser, so that the broken crystals are distinguishable with a weak lens. The layers vary in character in short distances. With the exception of a few of the Ostrea Patagonica, which appeared to have rolled down from the cliff above, no organic remains were found. The chief difference between these layers taken as a whole, and the upper beds both at the mouth of the river and on the coast northward, seems to lie in the occasional presence of more colouring matter, and in the supply having been intermittent; these characters, as we have seen, very gradually disappear in descending the valley, and this fact may perhaps be accounted for by the currents of a more open sea having blended together the sediment from a distant and intermittent source.

The coloured layers in the foregoing section rest on a mass, apparently of great thickness (but much hidden by the talus), of soft sandstone, almost composed of minute pebbles, from one-tenth to two-tenths of an inch in diameter, of the rocks (with the entire exception of the basaltic lava) composing the great boulders on the surface of the plain, and probably composing the neighbouring Cordillera. Five miles higher up the valley, and again thirty miles higher up (that is twenty miles from the nearest range of the Cordillera), the lower plain included within the upper escarpments, is formed, as seen on the banks of the river, of a nearly similar but finer-grained, more earthy, laminated sandstone, alternating with argillaceous beds, and containing numerous moderately sized pebbles of the same rocks, and some shells of the great Ostrea Patagonica. (I found at both places, but not in situ, quantities of coniferous and ordinary dicotyledonous silicified wood, which was examined for me by Mr. R. Brown.) As most of these shells had been rolled before being here embedded, their presence does not prove that the sandstone belongs to the great Patagonian tertiary formation, for they might have been redeposited in it, when the valley existed as a sea-strait; but as amongst the pebbles there were none of basalt, although the cliffs on both sides of the valley are composed of this rock, I believe that the sandstone does belong to this formation. At the highest point to which we ascended, twenty miles distant from the nearest slope of the Cordillera, I could see the horizontally zoned white beds, stretching under the black basaltic lava, close up to the mountains; so that the valley of the S. Cruz gives a fair idea of the constitution of the whole width of Patagonia.

BASALTIC LAVA OF THE S. CRUZ.

This formation is first met with sixty-seven miles from the mouth of the river; thence it extends uninterruptedly, generally but not exclusively on the northern side of the valley, close up to the Cordillera. The basalt is generally black and fine-grained, but sometimes grey and laminated; it contains some olivine, and high up the valley much glassy feldspar, where, also, it is often amygdaloidal; it is never highly vesicular, except on the sides of rents and on the upper and lower, spherically laminated surfaces. It is often columnar; and in one place I saw magnificent columns, each face twelve feet in width, with their interstices filled up with calcareous tuff. The streams rest conformably on the white sedimentary beds, but I nowhere saw the actual junction; nor did I anywhere see the white beds actually superimposed on the lava; but some way up the valley at the foot of the uppermost escarpments, they must be thus superimposed. Moreover, at the lowest point down the valley, where the streams thin out and terminate in irregular projections, the spaces or intervals between these projections are filled up to the level of the now denuded and gravel-capped surfaces of the plains, with the white-zoned sedimentary beds; proving that this matter continued to be deposited after the streams had flowed. Hence we may conclude that the basalt is contemporaneous with the upper parts of the great tertiary formation.

The lava where first met with is 130 feet in thickness: it there consists of two, three, or perhaps more streams, divided from each other by vesicular spheroids like those on the surface. From the streams having, as it appears, extended to different distances, the terminal points are of unequal heights. Generally the surface of the basalt is smooth them in one part high up the valley, it was so uneven and hummocky, that until I afterwards saw the streams extending continuously on both sides of the valley up to a height of about three thousand feet close to the Cordillera, I thought that the craters of eruption were probably close at hand. This hummocky surface I believe to have been caused by the crossing and heaping up of different streams. In one place, there were several rounded ridges about twenty feet in height, some of them as broad as high, and some broader, which certainly had been formed whilst the lava was fluid, for in transverse sections each ridge was seen to be concentrically laminated, and to be composed of imperfect columns radiating from common centres, like the spokes of wheels.

The basaltic mass where first met with is, as I have said, 130 feet in thickness, and, thirty-five miles higher up the valley, it increases to 322 feet. In the first fourteen and a half miles of this distance, the upper surface of the lava, judging from three measurements taken above the level of the river (of which the apparently very uniform inclination has been calculated from its total height at a point 135 miles from the mouth), slopes towards the Atlantic at an angle of only 0 degrees 7 minutes twenty seconds: this must be considered only as an approximate measurement, but it cannot be far wrong. Taking the whole thirty-five miles, the upper surface slopes at an angle of 0 degrees 10 minutes 53 seconds; but this result is of no value in showing the inclination of any one stream, for halfway between the two points of measurement, the surface suddenly rises between one hundred and two hundred feet, apparently caused by some of the uppermost streams having extended thus far and no farther. From the measurement made at these two points, thirty-five miles apart, the mean inclination of the sedimentary beds, over which the lava has flowed, is NOW (after elevation from under the sea) only 0 degrees 7 minutes 52 seconds: for the sake of comparison, it may be mentioned that the bottom of the present sea in a line from the mouth of the S. Cruz to the Falkland Islands, from a depth of seventeen fathoms to a depth of eighty-five fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between the beach and the depth of seventeen fathoms, the slope is greater. From a point about half-way up the valley, the basaltic mass rises more abruptly towards the foot of the Cordillera, namely, from a height of 1,204 feet, to about 3,000 feet above the sea.

This great deluge of lava is worthy, in its dimensions, of the great continent to which it belongs. The aggregate streams have flowed from the Cordillera to a distance (unparalleled, I believe, in any case yet known) of about one hundred geographical miles. Near their furthest extremity their total thickness is 130 feet, which increase thirty-five miles farther inland, as we have just seen, to 322 feet. The least inclination given by M. E. de Beaumont of the upper surface of a lava-stream, namely 0 degrees 30 minutes, is that of the great subaerial eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows that it must have flowed down a mean inclination of less than 0 degrees 20 minutes. ("Memoires pour servir" etc. pages 178 and 217.) But we now see that under the pressure of the sea, successive streams have flowed over a smooth bottom with a mean inclination of not more than 0 degrees 7 minutes 52 seconds; and that the upper surface of the terminal portion (over a space of fourteen and a half miles) has an inclination of not more than 0 degrees 7 minutes 20 seconds. If the elevation of Patagonia has been greater nearer the Cordillera than near the Atlantic (as is probable), then these angles are now all too large. I must repeat, that although the foregoing measurements, which were all carefully taken with the barometer, may not be absolutely correct, they cannot be widely erroneous.

Southward of the S. Cruz, the cliffs of the 840 feet plain extend to Coy Inlet, and owing to the naked patches of the white sediment, they are said on the charts to be "like the coast of Kent." At Coy Inlet the high plain trends inland, leaving flat-topped outliers. At Port Gallegos (latitude 51 degrees 35 minutes, and ninety miles south of S. Cruz), I am informed by Captain Sulivan, R.N., that there is a gravel-capped plain from two to three hundred feet in height, formed of numerous strata, some fine-grained and pale-coloured, like the upper beds at the mouth of the S. Cruz, others rather dark and coarser, so as to resemble gritstones or tuffs; these latter include rather large fragments of apparently decomposed volcanic rocks; there are, also, included layers of gravel. This formation is highly remarkable, from abounding with mammiferous remains, which have not as yet been examined by Professor Owen, but which include some large, but mostly small, species of Pachydermata, Edentata, and Rodentia. From the appearance of the pale-coloured, fine-grained beds, I was inclined to believe that they corresponded with the upper beds of the S. Cruz; but Professor Ehrenberg, who has examined some of the specimens, informs me that the included microscopical organisms are wholly different, being fresh and brackish-water forms. Hence the two to three hundred feet plain at Port Gallegos is of unknown age, but probably of subsequent origin to the great Patagonian tertiary formation.

EASTERN TIERRA DEL FUEGO.

Judging from the height, the general appearance, and the white colour of the patches visible on the hill sides, the uppermost plain, both on the north and western side of the Strait of Magellan, and along the eastern coast of Tierra del Fuego as far south as near Port St. Polycarp, probably belongs to the great Patagonian tertiary formation, These higher table- ranges are fringed by low, irregular, extensive plains, belonging to the boulder formation (Described in the "Geological Transactions" volume 6 page 415.), and composed of coarse unstratified masses, sometimes associated (as north of C. Virgin's) with fine, laminated, muddy sandstones. The cliffs in Sebastian Bay are 200 feet in height, and are composed of fine sandstones, often in curvilinear layers, including hard concretions of calcareous sandstone, and layers of gravel. In these beds there are fragments of wood, legs of crabs, barnacles encrusted with corallines still partially retaining their colour, imperfect fragments of a Pholas distinct from any known species, and of a Venus, approaching very closely to, but slightly different in form from, the V. lenticularis, a species living on the coast of Chile. Leaves of trees are numerous between the laminae of the muddy sandstone; they belong, as I am informed by Dr. J.D. Hooker, to three species of deciduous beech, different from the two species which compose the great proportion of trees in this forest-clad land. ("Botany of the Antarctic Voyage" page 212.) From these facts it is difficult to conjecture, whether we here see the basal part of the great Patagonian formation, or some later deposit.

SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.

Four out of the seven fossil shells, from St. Fe in Entre Rios, were found by M. d'Orbigny in the sandstone of the Rio Negro, and by me at San Josef. Three out of the six from San Josef are identical with those from Port Desire and S. Julian, which two places have together fifteen species, out of which three are common to both. Santa Cruz has seventeen species, out of which five are common to Port Desire and S. Julian. Considering the difference in latitude between these several places, and the small number of species altogether collected, namely thirty-six, I conceive the above proportional number of species in common, is sufficient to show that the lower fossiliferous mass belongs nearly, I do not say absolutely, to the same epoch. What this epoch may be, compared with the European tertiary stages, M. d'Orbigny will not pretend to determine. The thirty-six species (including those collected by myself and by M. d'Orbigny) are all extinct, or at least unknown; but it should be borne in mind, that the present coast consists of shingle, and that no one, I believe, has dredged here for shells; hence it is not improbable that some of the species may hereafter be found living. Some few of the species are closely related with existing ones; this is especially the case, according to M. d'Orbigny and Mr. Sowerby, with the Fusus Patagonicus; and, according to Mr. Sowerby, with the Pyrula, the Venus meridionalis, the Crepidula gregaria, and the Turritella ambulacrum, and T. Patagonica. At least three of the genera, namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby) Struthiolaria, are not found in this quarter of the world; and Trigonocelia is extinct. The evidence taken altogether indicates that this great tertiary formation is of considerable antiquity; but when treating of the Chilean beds, I shall have to refer again to this subject.