South American Geology

South American Geology


NAVIDAD.(I was guided to this locality by the Report on M.Gay's "Geological Researches" in the "Annales des Scienc.Nat." 1st series tome 28.)The Concepcion formation extends some distance northward, but how far Iknow not; for the next point at which I landed was at Navidad, 160 miles north of Concepcion, and 60 miles south of Valparaiso.The cliffs here are about eight hundred feet in height: they consist, wherever I could examine them, of fine-grained, yellowish, earthy sandstones, with ferruginous veins, and with concretions of hard calcareous sandstone.In one part, there were many pebbles of the common metamorphic porphyries of the Cordillera: and near the base of the cliff, I observed a single rounded boulder of greenstone, nearly a yard in diameter.I traced this sandstone formation beneath the superficial covering of gravel, for some distance inland: the strata are slightly inclined from the sea towards the Cordillera, which apparently has been caused by their having been accumulated against or round outlying masses of granite, of which some points project near the coast.The sandstone contains fragments of wood, either in the state of lignite or partially silicified, sharks' teeth, and shells in great abundance, both high up and low down the sea-cliffs.

Pectunculus and Oliva were most numerous in individuals, and next to them Turritella and Fusus.I collected in a short time, though suffering from illness, the following thirty-one species, all of which are extinct, and several of the genera do not now range (as we shall hereafter show) nearly so far south:--1.Gastridium cepa, G.B.Sowerby.

2.Monoceros, fragments of, considered by M.d'Orbigny as a new species.

3.Voluta alta, G.B.Sowerby (considered by M.d'Orbigny as distinct from the V.alta of Santa Cruz).

4.Voluta triplicata, G.B.Sowerby.

5.Oliva dimidiata, G.B.Sowerby.

6.Pleurotoma discors, G.B.Sowerby.

7.Pleurotoma turbinelloides, G.B.Sowerby.

8.Fusus subreflexus, G.B.Sowerby.

9.Fusus pyruliformis, G.B.Sowerby.

10.Fusus, allied to F.regularis (considered by M.d'Orbigny as a distinct species).

11.Turritella suturalis, G.B.Sowerby.

12.Turritella Patagonica, G.B.Sowerby (fragments of).

13.Trochus laevis, G.B.Sowerby.

14.Trochus collaris, G.B.Sowerby (considered by M.d'Orbigny as the young of the T.laevis).

15.Cassis monilifer, G.B.Sowerby.

16.Pyrula distans, G.B.Sowerby.

17.Triton verruculosus, G.B.Sowerby.

18.Sigaretus subglobosus, G.B.Sowerby.

19.Natica solida, G.B.Sowerby.(It is doubtful whether the Natica solida of S.Cruz is the same species with this.)20.Terebra undulifera, G.B.Sowerby.

21.Terebra costellata, G.B.Sowerby.

22.Bulla (fragments of).

23.Dentalium giganteum, do.

24.Dentalium sulcosum, do.

25.Corbis (?) laevigata, do.

26.Cardium multiradiatum, do.

27.Venus meridionalis, do.

28.Pectunculus dispar, (?) Desh.(considered by M.d'Orbigny as a distinct species).

29, 30.Cytheraea and Mactra, fragments of (considered by M.d'Orbigny as new species).

31.Pecten, fragments of.



>From Level of Sea to Surface of plain, 252 feet above sea, through levels F, E, D and C:

F.--Lower sandstone, with concretions and silicified bones, with fossil shells, all, or nearly all, extinct.

E.--Upper ferruginous sandstone, with numerous Balani, with fossil shells, all, or nearly all, extinct.

C and D.--Calcareous beds with recent shells.

A.--Stratified sand in a ravine, also with recent shells.)For more than two hundred miles northward of Navidad, the coast consists of plutonic and metamorphic rocks, with the exception of some quite insignificant superficial beds of recent origin.At Tonguay, twenty-five miles south of Coquimbo, tertiary beds recommence.I have already minutely described in the Second Chapter, the step-formed plains of Coquimbo, and the upper calcareous beds (from twenty to thirty feet in thickness)containing shells of recent species, but in different proportions from those on the beach.There remains to be described only the underlying ancient tertiary beds, represented in Figure 21 by the letters F and E:--I obtained good sections of bed F only in Herradura Bay: it consists of soft whitish sandstone, with ferruginous veins, some pebbles of granite, and concretionary layers of hard calcareous sandstone.These concretions are remarkable from the great number of large silicified bones, apparently of cetaceous animals, which they contain; and likewise of a shark's teeth, closely resembling those of the Carcharias megalodon.Shells of the following species, of which the gigantic Oyster and Perna are the most conspicuous, are numerously embedded in the concretions:--1.Bulla ambigua, d'Orbigny "Voyage" Pal.

2.Monoceros Blainvillii, d'Orbigny "Voyage" Pal.

3.Cardium auca, d'Orbigny "Voyage" Pal.

4.Panopaea Coquimbensis, d'Orbigny "Voyage" Pal.

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

6.Artemis ponderosa; Mr.Sowerby can find no distinguishing character between this fossil and the recent A.ponderosa; it is certainly an Artemis, as shown by the pallial impression.

7.Ostrea Patagonica (?); Mr.Sowerby can point out no distinguishing character between this species and that so eminently characteristic of the great Patagonian formation; but he will not pretend to affirm that they are identical.

8.Fragments of a Venus and Natica.

The cliffs on one side of Herradura Bay are capped by a mass of stratified shingle, containing a little calcareous matter, and I did not doubt that it belonged to the same recent formation with the gravel on the surrounding plains, also cemented by calcareous matter, until to my surprise, I found in the midst of it, a single thin layer almost entirely composed of the above gigantic oyster.

Charles Darwin