Thirdly, a dark coloured limestone with some quartz pebbles, from fifty to sixty feet in thickness, containing numerous silicified shells, presently to be enumerated.Fourthly, very compact, calcareous, jaspery sandstone, passing into (fifthly) a great bed, several hundred feet thick, of conglomerate, composed of pebbles of white, red, and purple porphyries, of sandstone and quartz, cemented by calcareous matter.I observed that some of the finer parts of this conglomerate were much indurated within a foot of a dike eight feet in width, and were rendered of a paler colour with the calcareous matter segregated into white crystallised particles; some parts were stained green from the colouring matter of the dike.Sixthly, a thick mass, obscurely stratified, of a red sedimentary stone or sandstone, full of crystalline calcareous matter, imperfect crystals of oxide of iron, and I believe of feldspar, and therefore closely resembling some of the highly metamorphosed beds at Arqueros: this bed was capped by, and appeared to pass in its upper part into, rocks similarly coloured, containing calcareous matter, and abounding with minute crystals, mostly elongated and glassy, of reddish albite.Seventhly, a conformable stratum of fine reddish porphyry with large crystals of (albitic?) feldspar; probably a submarine lava.Eighthly, another conformable bed of green porphyry, with specks of green earth and cream-coloured crystals of feldspar.I believe that there are other superincumbent crystalline strata and submarine lavas, but I had not time to examine them.
The upper beds in this section probably correspond with parts of the great gypseous formation; and the lower beds of red sandstone conglomerate and fossiliferous limestone no doubt are the equivalents of the Hippurite stratum, seen in descending from Arqueros to Pluclaro, which there lies conformably upon the porphyritic conglomerate formation.The fossils found in the third bed, consist of:--Pecten Dufreynoyi, d'Orbigny, "Voyage, Part Pal."This species, which occurs here in vast numbers, according to M.D'Orbigny, resembles certain cretaceous forms.
Ostrea hemispherica, d'Orbigny, "Voyage" etc.
Also resembles, according to the same author, cretaceous forms.
Terebratula aenigma, d'Orbigny, "Voyage" etc.(Pl.22 Figures 10-12.)Is allied, according to M.d'Orbigny, to T.concinna from the Forest Marble.A series of this species, collected in several localities hereafter to be referred to, has been laid before Professor Forbes; and he informs me that many of the specimens are almost undistinguishable from our oolitic T.
tetraedra, and that the varieties amongst them are such as are found in that variable species.Generally speaking, the American specimens of T.
aenigma may be distinguished from the British T.tetraedra, by the surface having the ribs sharp and well-defined to the beak, whilst in the British species they become obsolete and smoothed down; but this difference is not constant.Professor Forbes adds, that, possibly, internal characters may exist, which would distinguish the American species from its European allies.
Spirifer linguiferoides, E.Forbes.
Professor Forbes states that this species is very near to S.linguifera of Phillips (a carboniferous limestone fossil), but probably distinct.M.
d'Orbigny considers it as perhaps indicating the Jurassic period.
Ammonites, imperfect impression of.
M.Domeyko has sent to France a collection of fossils, which, I presume, from the description given, must have come from the neighbourhood of Arqueros; they consist of:--Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal.
Ostrea hemispherica, d'Orbigny, "Voyage" Part Pal.
Turritella Andii, d'Orbigny, "Voyage" Part Pal.(Pleurotomaria Humboldtii of Von Buch).
Hippurites Chilensis, d'Orbigny, "Voyage" Part Pal.
The specimens of this Hippurite, as well as those I collected in my descent from Arqueros, are very imperfect; but in M.d'Orbigny's opinion they resemble, as does the Turritella Andii, cretaceous (upper greensand) forms.
Nautilus Domeykus, d'Orbigny, "Voyage" Part Pal.
Terebratula aenigma, d'Orbigny, "Voyage" Part Pal.
Terebratula ignaciana, d'Orbigny, "Voyage" Part Pal.
This latter species was found by M.Domeyko in the same block of limestone with the T.aenigma.According to M.d'Orbigny, it comes near to T.
ornithocephala from the Lias.A series of this species collected at Guasco, has been examined by Professor E.Forbes, and he states that it is difficult to distinguish between some of the specimens and the T.hastata from the mountain limestone; and that it is equally difficult to draw a line between them and some Marlstone Terebratulae.Without a knowledge of the internal structure, it is impossible at present to decide on their identity with analogous European forms.
The remarks given on the several foregoing shells, show that, in M.
