The second bed B is about fifteen feet in thickness, but towards both extremities of the cliff (not included in the diagram) it either thins out and dies away, or passes insensibly into an overlying bed of gravel.It consists of red, tough clayey mud, with minute linear cavities; it is marked with faint horizontal shades of colour; it includes a few pebbles, and rarely a minute particle of shell: in one spot, the dermal armour and a few bones of a Dasypoid quadruped were embedded in it: it fills up furrows in the underlying gravel.With the exception of the few pebbles and particles of shells, this bed resembles the true Pampean mud; but it still more closely resembles the clayey flats (mentioned in the First Chapter)separating the successively rising parallel ranges of sand-dunes.
The bed C is of stratified gravel, like the lowest one; it fills up furrows in the underlying red mud, and is sometimes interstratified with it, and sometimes insensibly passes into it; as the red mud thins out, this upper gravel thickens.Shells are more numerous in it than in the lower gravel;but the bones, though some are still present, are less numerous.In one part, however, where this gravel and the red mud passed into each other, Ifound several bones and a tolerably perfect head of the Megatherium.Some of the large Volutas, though embedded in the gravel-bed C, were filled with the red mud, including great numbers of the little recent Paludestrina australis.These three lower beds are covered by an unconformable mantle Dof stratified sandy earth, including many pebbles of quartz, pumice and phonolite, land and sea-shells.
M.d'Orbigny has been so obliging as to name for me the twenty species of Mollusca embedded in the two gravel beds: they consist of:--1.Volutella angulata, d'Orbigny, "Voyage" Mollusq.and Pal.
2.Voluta Braziliana, Sol 3.Olicancilleria Braziliensis d'Orbigny.
4.Olicancilleria auricularia, d'Orbigny.
5.Olivina puelchana, d'Orbigny.
6.Buccinanops cochlidium, d'Orbigny.
7.Buccinanops globulosum, d'Orbigny.
8.Colombella sertulariarum, d'Orbigny.
9.Trochus Patagonicus, and var.of ditto, d'Orbigny.
10.Paludestrina Australis, d'Orbigny.
11.Fissurella Patagonica, d'Orbigny.
12.Crepidula muricata, Lam.
13.Venus purpurata, Lam.
14.Venus rostrata, Phillippi.
15.Mytilus Darwinianus, d'Orbigny.
16.Nucula semiornata, d'Orbigny.
17.Cardita Patagonica, d'Orbigny.
18.Corbula Patagonica (?), d'Orbigny.
19.Pecten tethuelchus, d'Orbigny.
20.Ostrea puelchana, d'Orbigny.
21.A living species of Balanus.
22 and 23.An Astrae and encrusting Flustra, apparently identical with species now living in the bay.
All these shells now live on this coast, and most of them in this same bay.
I was also struck with the fact, that the proportional numbers of the different kinds appeared to be the same with those now cast up on the beach: in both cases specimens of Voluta, Crepidula, Venus, and Trochus are the most abundant.Four or five of the species are the same with the upraised shells on the Pampas near Buenos Ayres.All the specimens have a very ancient and bleached appearance, and do not emit, when heated, an animal odour: some of them are changed throughout into a white, soft, fibrous substance; others have the space between the external walls, either hollow, or filled up with crystalline carbonate of lime.(A Bulinus, mentioned in the Introduction to the "Fossil Mammalia" in the "Zoology of the Voyage of the 'Beagle'" has so much fresher an appearance, than the marine species, that I suspect it must have fallen amongst the others, and been collected by mistake.)The remains of the extinct mammiferous animals, from the two gravel beds have been described by Professor Owen in the "Zoology of the Voyage of the 'Beagle':" they consist of, 1st, one nearly perfect head and three fragments of heads of the Megatherium Cuvierii; 2nd, a lower jaw of Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th, fragments of a head of some gigantic Edental quadruped; 5th, an almost entire skeleton of the great Scelidotherium leptocephalum, with most of the bones, including the head, vertebrae, ribs, some of the extremities to the claw-bone, and even, as remarked by Professor Owen, the knee-cap, all nearly in their proper relative positions; 6th, fragments of the jaw and a separate tooth of a Toxodon, belonging either to T.Platensis, or to a second species lately discovered near Buenos Ayres; 7th, a tooth of Equus curvidens; 8th, tooth of a Pachyderm, closely allied to Palaeotherium, of which parts of the head have been lately sent from Buenos Ayres to the British Museum; in all probability this pachyderm is identical with the Macrauchenia Patagonica from Port S.Julian, hereafter to be referred to.
