Geological Observations on South America Part 13

10. Mactra cecileana, d'Orbigny, "Voyage, Part Pal."

11. Mactra Araucana, d'Orbigny, "Voyage, Part Pal."

12. Arca Araucana, d'Orbigny, "Voyage, Part Pal."

13. Nucula Largillierti, d'Orbigny, "Voyage, Part Pal."

14. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."

During a second visit of the "Beagle" to Concepcion, Mr. Kent collected for me some silicified wood and sh.e.l.ls out of the concretions in the sandstone from Tome, situated a short distance north of Lirguen. They consist of:--

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

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

3. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."

4. Pecten, fragments of, probably two species, but too imperfect for description.

5. Baculites v.a.g.i.n.a, E. Forbes.

6. Nautilus d'Orbignya.n.u.s, E. Forbes.

Besides these sh.e.l.ls, Captain Belcher found here an Ammonite, nearly three feet in diameter, and so heavy that he could not bring it away; fragments are deposited at Haslar Hospital: he also found the silicified vertebrae of some very large animal. ("Zoology of Captain Beechey's Voyage" page 163.) From the ident.i.ty in mineralogical nature of the rocks, and from Captain Belcher's minute description of the coast between Lirguen and Tome, the fossiliferous concretions at this latter place certainly belong to the same formation with the beds examined by myself at Lirguen; and these again are undoubtedly the same with the strata of Quiriquina; moreover; the three first of the sh.e.l.ls from Tome, though a.s.sociated in the same concretions with the Baculite, are identical with the species from Quiriquina. Hence all the sandstone and lignitiferous beds in this neighbourhood certainly belong to the same formation. Although the generic character of the Quiriquina fossils naturally led M. d'Orbigny to conceive that they were of tertiary origin, yet as we now find them a.s.sociated with the Baculites v.a.g.i.n.a and with an Ammonite, we must, in the opinion of M. d'Orbigny, and if we are guided by the a.n.a.logy of the northern hemisphere, rank them in the Cretaceous system. Moreover, the Baculites v.a.g.i.n.a, which is in a tolerable state of preservation, appears to Professor E. Forbes certainly to be identical with a species, so named by him, from Pondicherry in India; where it is a.s.sociated with numerous decidedly cretaceous species, which approach most nearly to Lower Greensand or Neocomian forms: this fact, considering the vast distance between Chile and India, is truly surprising.

Again, the Nautilus d'Orbignya.n.u.s, as far as its imperfect state allows of comparison, resembles, as I am informed by Professor Forbes, both in its general form and in that of its chambers, two species from the Upper Greensand. It may be added that every one of the above-named genera from Quiriquina, which have an apparently tertiary character, are found in the Pondicherry strata. There are, however, some difficulties on this view of the formations at Concepcion being cretaceous, which I shall afterwards allude to; and I will here only state that the Cardium auca is found also at Coquimbo, the beds at which place, there can be no doubt, are tertiary.

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 I know 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 acc.u.mulated against or round outlying ma.s.ses 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 sh.e.l.ls 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. Ca.s.sis 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.

COQUIMBO.

(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO.

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 sh.e.l.ls, all, or nearly all, extinct.

E.--Upper ferruginous sandstone, with numerous Balani, with fossil sh.e.l.ls, all, or nearly all, extinct.

C and D.--Calcareous beds with recent sh.e.l.ls.

A.--Stratified sand in a ravine, also with recent sh.e.l.ls.)

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 sh.e.l.ls 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. Sh.e.l.ls 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 distinguis.h.i.+ng 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 distinguis.h.i.+ng 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 ma.s.s of stratified s.h.i.+ngle, 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.

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 ma.s.s, 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 sh.e.l.ls 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 distinguis.h.i.+ng character between this fossil, as far as its not very perfect condition allows of comparison, and the recent species.

9. Bala.n.u.s Coquimbensis, G.B. Sowerby.

10. Bala.n.u.s 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 ma.s.s 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 pa.s.sage between them; but as all the species are recent in the bed D, whilst the most characteristic sh.e.l.ls 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 pa.s.sage 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 Clerya.n.u.s, d'Orbigny "Voyage" Pal.

2. Fusus pet.i.tia.n.u.s, d'Orbigny "Voyage" Pal.

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