South American Geology

第82章 ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND

At a little distance inland, I obtained several sections of the bed E, which, though different in appearance from the lower bed F, belongs to the same formation: it consists of a highly ferruginous sandy mass, almost composed, like the lowest bed at Port S.Julian, of fragments of Balanidae;it includes some pebbles, and layers of yellowish-brown mudstone.The embedded shells consist of:--1.Monoceros Blainvillii, d'Orbigny "Voyage" Pal.

2.Monoceros ambiguus, G.B.Sowerby.

3.Anomia alternans, G.B.Sowerby.

4.Pecten rudis, G.B.Sowerby.

5.Perna Gaudichaudi, d'Orbigny "Voyage" Pal.

6.Ostrea Patagonica (?), d'Orbigny "Voyage" Pal.

7.Ostrea, small species, in imperfect state; it appeared to me like a small kind now living in, but very rare in the bay.

8.Mytilus Chiloensis; Mr.Sowerby can find no distinguishing character between this fossil, as far as its not very perfect condition allows of comparison, and the recent species.

9.Balanus Coquimbensis, G.B.Sowerby.

10.Balanus psittacus? King.This appears to Mr.Sowerby and myself identical with a very large and common species now living on the coast.

The uppermost layers of this ferrugino-sandy mass are conformably covered by, and impregnated to the depth of several inches with, the calcareous matter of the bed D called losa: hence I at one time imagined that there was a gradual passage between them; but as all the species are recent in the bed D, whilst the most characteristic shells of the uppermost layers of E are the extinct Perna, Pecten, and Monoceros, I agree with M.d'Orbigny, that this view is erroneous, and that there is only a mineralogical passage between them, and no gradual transition in the nature of their organic remains.Besides the fourteen species enumerated from these two lower beds, M.d'Orbigny has described ten other species given to him from this locality; namely:--1.Fusus Cleryanus, d'Orbigny "Voyage" Pal.

2.Fusus petitianus, d'Orbigny "Voyage" Pal.

3.Venus hanetiana, d'Orbigny "Voyage" Pal.

4.Venus incerta (?) d'Orbigny "Voyage" Pal.

5.Venus Cleryana, d'Orbigny "Voyage" Pal.

6.Venus petitiana, d'Orbigny "Voyage" Pal.

7.Venus Chilensis, d'Orbigny "Voyage" Pal.

8.Solecurtus hanetianus, d'Orbigny "Voyage" Pal.

9.Mactra auca, d'Orbigny "Voyage" Pal.

10.Oliva serena, d'Orbigny "Voyage" Pal.

Of these twenty-four shells, all are extinct, except, according to Mr.

Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and probably the great Balanus.

COQUIMBO TO COPIAPO.

A few miles north of Coquimbo, I met with the ferruginous, balaniferous mass E with many silicified bones; I was informed that these silicified bones occur also at Tonguay, south of Coquimbo: their number is certainly remarkable, and they seem to take the place of the silicified wood, so common on the coast-formations of Southern Chile.In the valley of Chaneral, I again saw this same formation, capped with the recent calcareous beds.I here left the coast, and did not see any more of the tertiary formations, until descending to the sea at Copiapo: here in one place I found variously coloured layers of sand and soft sandstone, with seams of gypsum, and in another place, a comminuted shelly mass, with layers of rotten-stone and seams of gypsum, including many of the extinct gigantic oyster: beds with these oysters are said to occur at English Harbour, a few miles north of Copiapo.

COAST OF PERU.

With the exception of deposits containing recent shells and of quite insignificant dimensions, no tertiary formations have been observed on this coast, for a space of twenty-two degrees of latitude north of Copiapo, until coming to Payta, where there is said to be a considerable calcareous deposit: a few fossils have been described by M.d'Orbigny from this place, namely:--1.Rostellaria Gaudichaudi, d'Orbigny "Voyage" Pal.

2.Pectunculus Paytensis, d'Orbigny "Voyage" Pal.

3.Venus petitiana, d'Orbigny "Voyage" Pal.

4.Ostrea Patagonica? This great oyster (of which specimens have been given me) cannot be distinguished by Mr.Sowerby from some of the varieties from Patagonia; though it would be hazardous to assert it is the same with that species, or with that from Coquimbo.

CONCLUDING REMARKS.

The formations described in this chapter, have, in the case of Chiloe and probably in that of Concepcion and Navidad, apparently been accumulated in troughs formed by submarine ridges extending parallel to the ancient shores of the continent; in the case of the islands of Mocha and Huafo it is highly probable, and in that of Ypun and Lemus almost certain, that they were accumulated round isolated rocky centres or nuclei, in the same manner as mud and sand are now collecting round the outlying islets and reefs in the West Indian Archipelago.Hence, I may remark, it does not follow that the outlying tertiary masses of Mocha and Huafo were ever continuously united at the same level with the formations on the mainland, though they may have been of contemporaneous origin, and been subsequently upraised to the same height.In the more northern parts of Chile, the tertiary strata seem to have been separately accumulated in bays, now forming the mouths of valleys.

Charles Darwin

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