Geological Observations on South America Part 12

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 (lat.i.tude 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 coa.r.s.er, 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 coa.r.s.e unstratified ma.s.ses, sometimes a.s.sociated (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 sh.e.l.ls, 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 lat.i.tude 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 ma.s.s 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 s.h.i.+ngle, and that no one, I believe, has dredged here for sh.e.l.ls; 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, Cra.s.satella, 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.

The white pumiceous mudstone, with its abundant gypsum, belongs to the same general epoch with the underlying fossiliferous ma.s.s, as may be inferred from the sh.e.l.ls included in the intercalated layers at Nuevo Gulf, S.

Julian, and S. Cruz. Out of the twenty-seven marine microscopic structures found by Professor Ehrenberg in the specimens from S. Julian and Port Desire, ten are common to these two places: the three found at Nuevo Gulf are distinct. I have minutely described this deposit, from its remarkable characters and its wide extension. From Coy Inlet to Port Desire, a distance of 230 miles, it is certainly continuous; and I have reason to believe that it likewise extends to the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we have, also, seen that a single layer occurs at the Rio Negro. At Port S. Julian it is from eight to nine hundred feet in thickness; and at S. Cruz it extends, with a slightly altered character, up to the Cordillera. From its microscopic structure, and from its a.n.a.logy with other formations in volcanic districts, it must be considered as originally of volcanic origin: it may have been formed by the long-continued attrition of vast quant.i.ties of pumice, or judging from the manner in which the ma.s.s becomes, in ascending the valley of S. Cruz, divided into variously coloured layers, from the long-continued eruption of clouds of fine ashes. In either case, we must conclude, that the southern volcanic orifices of the Cordillera, now in a dormant state, were at about this period over a wide s.p.a.ce, and for a great length of time, in action.

We have evidence of this fact, in the lat.i.tude of the Rio Negro, in the sandstone-conglomerate with pumice, and demonstrative proof of it, at S.

Cruz, in the vast deluges of basaltic lava: at this same tertiary period, also, there is distinct evidence of volcanic action in Western Banda Oriental.

The Patagonian tertiary formation extends continuously, judging from fossils alone, from S. Cruz to near the Rio Colorado, a distance of above six hundred miles, and reappears over a wide area in Entre Rios and Banda Oriental, making a total distance of 1,100 miles; but this formation undoubtedly extends (though no fossils were collected) far south of the S.

Cruz, and, according to M. d'Orbigny, 120 miles north of St. Fe. At S. Cruz we have seen that it extends across the continent; being on the coast about eight hundred feet in thickness (and rather more at S. Julian), and rising with the contemporaneous lava-streams to a height of about three thousand feet at the base of the Cordillera. It rests, wherever any underlying formation can be seen, on plutonic and metamorphic rocks. Including the newer Pampean deposit, and those strata in Eastern Tierra del Fuego of doubtful age, as well as the boulder formation, we have a line of more than twenty-seven degrees of lat.i.tude, equal to that from the Straits of Gibraltar to the south of Iceland, continuously composed of tertiary formations. Throughout this great s.p.a.ce the land has been upraised, without the strata having been in a single instance, as far as my means of observation went, unequally tilted or dislocated by a fault.

TERTIARY FORMATIONS ON THE WEST COAST.

CHONOS ARCHIPELAGO.

The numerous islands of this group, with the exception of Lemus, Ypun, consist of metamorphic schists; these two islands are formed of softish grey and brown, fusible, often laminated sandstones, containing a few pebbles, fragments of black lignite, and numerous mammillated concretions of hard calcareous sandstone. Out of these concretions at Ypun (lat.i.tude 40 degrees 30 minutes S.), I extracted the four following extinct species of sh.e.l.ls:--

1. Turritella suturalis, G.B. Sowerby (also Navidad).

2. Sigaretus subglobosus, G.B. Sowerby (also Navidad).

3. Cytheraea (?) sulculosa (?), G.B. Sowerby (also Chiloe and Huafo?).

4. Voluta, fragments of.

In the northern parts of this group there are some cliffs of gravel and of the boulder formation. In the southern part (at P. Andres in Tres Montes), there is a volcanic formation, probably of tertiary origin. The lavas attain a thickness of from two to three hundred feet; they are extremely variable in colour and nature, being compact, or brecciated, or cellular, or amygdaloidal with zeolite, agate and bole, or porphyritic with gla.s.sy albitic feldspar. There is also much imperfect rubbly pitchstone, with the interstices charged with powdery carbonate of lime apparently of contemporaneous origin. These lavas are conformably a.s.sociated with strata of breccia and of brown tuff containing lignite. The whole ma.s.s has been broken up and tilted at an angle of 45 degrees, by a series of great volcanic dikes, one of which was thirty yards in breadth. This volcanic formation resembles one, presently to be described, in Chiloe.

HUAFO.

This island lies between the Chonos and Chiloe groups: it is about eight hundred feet high, and perhaps has a nucleus of metamorphic rocks. The strata which I examined consisted of fine-grained muddy sandstones, with fragments of lignite and concretions of calcareous sandstone. I collected the following extinct sh.e.l.ls, of which the Turritella was in great numbers:--

1. Bulla cosmophila, G.B. Sowerby.

2. Pleurotoma subaequalis, G.B. Sowerby.

3. Fusus clerya.n.u.s, d'Orbigny, "Voyage Pal." (also at Coquimbo).

4. Triton leucostomoides, G.B. Sowerby.

5. Turritella Chilensis, G.B. Sowerby (also Mocha).

6. Venus, probably a distinct species, but very imperfect.

7. Cytheraea (?) sulculosa (?), probably a distinct species, but very imperfect.

8. Dentalium majus, G.B. Sowerby.

CHILOE.

This fine island is about one hundred miles in length. The entire southern part, and the whole western coast, consists of mica-schist, which likewise is seen in the ravines of the interior. The central mountains rise to a height of 3,000 feet, and are said to be partly formed of granite and greenstone: there are two small volcanic districts. The eastern coast, and large parts of the northern extremity of the island are composed of gravel, the boulder formation, and underlying horizontal strata. The latter are well displayed for twenty miles north and south of Castro; they vary in character from common sandstone to fine-grained, laminated mudstones: all the specimens which I examined are easily fusible, and some of the beds might be called volcanic grit-stones. These latter strata are perhaps related to a ma.s.s of columnar trachyte which occurs behind Castro. The sandstone occasionally includes pebbles, and many fragments and layers of lignite; of the latter, some are apparently formed of wood and others of leaves: one layer on the N.W. side of Lemuy is nearly two feet in thickness. There is also much silicified wood, both common dicotyledonous and coniferous: a section of one specimen in the direction of the medullary rays has, as I am informed by Mr. R. Brown, the discs in a double row placed alternately, and not opposite as in the true Araucaria. I found marine remains only in one spot, in some concretions of hard calcareous sandstone: in several other districts I have observed that organic remains were exclusively confined to such concretions; are we to account for this fact, by the supposition that the sh.e.l.ls lived only at these points, or is it not more probable that their remains were preserved only where concretions were formed? The sh.e.l.ls here are in a bad state, they consist of:--

1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d'Orbigny's opinion).

2. Natica striolata, G.B. Sowerby.

3. Natica (?) pumila, G.B. Sowerby.

4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and Huafo?).

At the northern extremity of the island, near S. Carlos, there is a large volcanic formation, between five and seven hundred feet in thickness. The commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal with calcareous spar and bole: most even of the darkest varieties fuse into a pale-coloured gla.s.s. The next commonest variety is a rubbly, rarely well characterised pitchstone (fusing into a white gla.s.s) which pa.s.ses in the most irregular manner into stony grey lavas. This pitchstone, as well as some purple claystone porphyry, certainly flowed in the form of streams.

These various lavas often pa.s.s, at a considerable depth from the surface, in the most abrupt and singular manner into wacke. Great ma.s.ses of the solid rock are brecciated, and it was generally impossible to discover whether the recementing process had been an igneous or aqueous action. (In a cliff of the hardest fragmentary ma.s.s, I found several tortuous, vertical veins, varying in thickness from a few tenths of an inch to one inch and a half, of a substance which I have not seen described. It is glossy, and of a brown colour; it is thinly laminated, with the laminae transparent and elastic; it is a little harder than calcareous spar; it is infusible under the blowpipe, sometimes decrepitates, gives out water, curls up, blackens, and becomes magnetic. Borax easily dissolves a considerable quant.i.ty of it, and gives a gla.s.s tinged with green. I have no idea what its true nature is. On first seeing it, I mistook it for lignite!) The beds are obscurely separated from each other; they are sometimes parted by seams of tuff and layers of pebbles. In one place they rested on, and in another place were capped by, tuffs and girt-stones, apparently of submarine origin.

The neighbouring peninsula of Lacuy is almost wholly formed of tufaceous deposits, connected probably in their origin with the volcanic hills just described. The tuffs are pale-coloured, alternating with laminated mudstones and sandstones (all easily fusible), and pa.s.sing sometimes into fine-grained white beds strikingly resembling the great upper infusorial deposit of Patagonia, and sometimes into brecciolas with pieces of pumice in the last stage of decay; these again pa.s.s into ordinary coa.r.s.e breccias and conglomerates of hard rocks. Within very short distances, some of the finer tuffs often pa.s.sed into each other in a peculiar manner, namely, by irregular polygonal concretions of one variety increasing so much and so suddenly in size, that the second variety, instead of any longer forming the entire ma.s.s, was left merely in thin veins between the concretions. In a straight line of cliffs, at Point Tenuy, I examined the following remarkable section (Figure 19):--

(FIGURE 19.)

On the left hand, the lower part (AA) consists of regular, alternating strata of brown tuffs and greenish laminated mudstone, gently inclined to the right, and conformably covered by a ma.s.s (B left) of a white, tufaceous and brecciolated deposit. On the right hand, the whole cliff (BB right) consists of the same white tufaceous matter, which on this side presents scarcely a trace of stratification, but to the left becomes very gradually and rather indistinctly divided into strata quite conformable with the underlying beds (AA): moreover, a few hundred yards further to the left, where the surface has been less denuded, the tufaceous strata (B left) are conformably covered by another set of strata, like the underlying ones (AA) of this section. In the middle of the diagram, the beds (AA) are seen to be abruptly cut off, and to abut against the tufaceous non-stratified ma.s.s; but the line of junction has been accidentally not represented steep enough, for I particularly noticed that before the beds had been tilted to the right, this line must have been nearly vertical. It appears that a current of water cut for itself a deep and steep submarine channel, and at the same time or afterwards filled it up with the tufaceous and brecciolated matter, and spread the same over the surrounding submarine beds; the matter becoming stratified in these more distant and less troubled parts, and being moreover subsequently covered up by other strata (like AA) not shown in the diagram. It is singular that three of the beds (of AA) are prolonged in their proper direction, as represented, beyond the line of junction into the white tufaceous matter: the prolonged portions of two of the beds are rounded; in the third, the terminal fragment has been pushed upwards: how these beds could have been left thus prolonged, I will not pretend to explain. In another section on the opposite side of a promontory, there was at the foot of this same line of junction, that is at the bottom of the old submarine channel, a pile of fragments of the strata (AA), with their interstices filled up with white tufaceous matter: this is exactly what might have been antic.i.p.ated under such circ.u.mstances.

(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND CONCRETIONARY ZONES IN A Ma.s.s OF TUFF.)

The various tufaceous and other beds at this northern end of Chiloe probably belong to about the same age with those near Castro, and they contain, as there, many fragments of black lignite and of silicified and pyritous wood, often embedded close together. They also contain many and singular concretions: some are of hard calcareous sandstone, in which it would appear that broken volcanic crystals and scales of mica have been better preserved (as in the case of the organic remains near Castro) than in the surrounding ma.s.s. Other concretions in the white brecciola are of a hard, ferruginous, yet fusible, nature; they are as round as cannon-b.a.l.l.s, and vary from two or three inches to two feet in diameter; their insides generally consist either of fine, scarcely coherent volcanic sand (The frequent tendency in iron to form hollow concretions or sh.e.l.l containing incoherent matter is singular; D'Aubuisson ("Traite de Geogn." tome 1 page 318) remarks on this circ.u.mstance.), or of an argillaceous tuff; in this latter case, the external crust was quite thin and hard. Some of these spherical b.a.l.l.s were encircled in the line of their equators, by a necklace-like row of smaller concretions. Again there were other concretions, irregularly formed, and composed of a hard, compact, ash- coloured stone, with an almost porcelainous fracture, adhesive to the tongue, and without any calcareous matter. These beds are, also, interlaced by many veins, containing gypsum, ferruginous matter, calcareous spar, and agate. It was here seen with remarkable distinctness, how intimately concretionary action and the production of fissures and veins are related together. Figure 20 is an accurate representation of a horizontal s.p.a.ce of tuff, about four feet long by two and a half in width: the double lines represent the fissures partially filled with oxide of iron and agate: the curvilinear lines show the course of the innumerable, concentric, concretionary zones of different shades of colour and of coa.r.s.eness in the particles of tuff. The symmetry and complexity of the arrangement gave the surface an elegant appearance. It may be seen how obviously the fissures determine (or have been determined by) the shape, sometimes of the whole concretion, and sometimes only of its central parts. The fissures also determine the curvatures of the long undulating zones of concretionary action. From the varying composition of the veins and concretions, the amount of chemical action which the ma.s.s has undergone is surprisingly great; and it would likewise appear from the difference in size in the particles of the concretionary zones, that the ma.s.s, also, has been subjected to internal mechanical movements.

In the peninsula of Lacuy, the strata over a width of four miles have been upheaved by three distinct, and some other indistinct, lines of elevation, ranging within a point of north and south. One line, about two hundred feet in height, is regularly anticlinal, with the strata dipping away on both sides, at an angle of 15 degrees, from a central "valley of elevation,"

about three hundred yards in width. A second narrow steep ridge, only sixty feet high, is uniclinal, the strata throughout dipping westward; those on both flanks being inclined at an angle of from ten to fifteen degrees; whilst those on the ridge dip in the same direction at an angle of between thirty and forty degrees. This ridge, traced northwards, dies away; and the beds at its terminal point, instead of dipping westward, are inclined 12 degrees to the north. This case interested me, as being the first in which I found in South America, formations perhaps of tertiary origin, broken by lines of elevation.

VALDIVIA: ISLAND OF MOCHA.

The formations of Chiloe seem to extend with nearly the same character to Valdivia, and for some leagues northward of it: the underlying rocks are micaceous schists, and are covered up with sandstone and other sedimentary beds, including, as I was a.s.sured, in many places layers of lignite. I did not land on Mocha (lat.i.tude 38 degrees 20 minutes), but Mr. Stokes brought me specimens of the grey, fine-grained, slightly calcareous sandstone, precisely like that of Huafo, containing lignite and numerous Turritellae.

The island is flat topped, 1,240 feet in height, and appears like an outlier of the sedimentary beds on the mainland. The few sh.e.l.ls collected consist of:--

1. Turritella Chilensis, G.B. Sowerby (also at Huafo).

2. Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad, but probably different.

3. Venus, fragments of.

CONCEPCION.

Sailing northward from Valdivia, the coast-cliffs are seen, first to a.s.sume near the R. Tolten, and thence for 150 miles northward, to be continued with the same mineralogical characters, immediately to be described at Concepcion. I heard in many places of beds of lignite, some of it fine and glossy, and likewise of silicified wood; near the Tolten the cliffs are low, but they soon rise in height; and the horizontal strata are prolonged, with a nearly level surface, until coming to a more lofty tract between points Rumena and Lavapie. Here the beds have been broken up by at least eight or nine parallel lines of elevation, ranging E. or E.N.E. and W. or W.S.W. These lines can be followed with the eye many miles into the interior; they are all uniclinal, the strata in each dipping to a point between S. and S.S.E. with an inclination in the central lines of about forty degrees, and in the outer ones of under twenty degrees. This band of symmetrically troubled country is about eight miles in width.

The island of Quiriquina, in the Bay of Concepcion, is formed of various soft and often ferruginous sandstones, with bands of pebbles, and with the lower strata sometimes pa.s.sing into a conglomerate resting on the underlying metamorphic schists. These beds include subordinate layers of greenish impure clay, soft micaceous and calcareous sandstones, and reddish friable earthy matter with white specks like decomposed crystals of feldspar; they include, also, hard concretions, fragments of sh.e.l.ls, lignite, and silicified wood. In the upper part they pa.s.s into white, soft sediments and brecciolas, very like those described at Chiloe; as indeed is the whole formation. At Lirguen and other places on the eastern side of the bay, there are good sections of the lower sandstones, which are generally ferruginous, but which vary in character, and even pa.s.s into an argillaceous nature; they contain hard concretions, fragments of lignite, silicified wood, and pebbles (of the same rocks with the pebbles in the sandstones of Quiriquina), and they alternate with numerous, often very thin layers of imperfect coal, generally of little specific gravity. The main bed here is three feet thick; and only the coal of this one bed has a glossy fracture. Another irregular, curvilinear bed of brown, compact lignite, is remarkable for being included in a ma.s.s of coa.r.s.e gravel. These imperfect coals, when placed in a heap, ignite spontaneously. The cliffs on this side of the bay, as well as on the island of Quiriquina, are capped with red friable earth, which, as stated in the Second Chapter, is of recent formation. The stratification in this neighbourhood is generally horizontal; but near Lirguen the beds dip N.W. at an angle of 23 degrees; near Concepcion they are also inclined: at the northern end of Quiriquina they have been tilted at an angle of 30 degrees, and at the southern end at angles varying from 15 degrees to 40 degrees: these dislocations must have taken place under the sea.

A collection of sh.e.l.ls, from the island of Quiriquina, has been described by M. d'Orbigny: they are all extinct, and from their generic character, M.

d'Orbigny inferred that they were of tertiary origin: they consist of:--

1. Scalaria Chilensis, d'Orbigny, "Voyage, Part Pal."

2. Natica Araucana, d'Orbigny, "Voyage, Part Pal."

3. Natica australis, d'Orbigny, "Voyage, Part Pal."

4. Fusus difficilis, d'Orbigny, "Voyage, Part Pal."

5. Pyrula longirostra, d'Orbigny, "Voyage, Part Pal."

6. Pleurotoma Araucana, d'Orbigny, "Voyage, Part Pal."

7. Cardium auca, d'Orbigny, "Voyage, Part Pal."

8. Cardium acuticostatum, d'Orbigny, "Voyage, Part Pal."

9. Venus auca, d'Orbigny, "Voyage, Part Pal."