15th.Grand thick bed, of a very hard, yellowish-white rock, with a crystalline feldspathic base, including large crystals of white feldspar, many little cavities mostly full of soft ferruginous matter, and numerous hexagonal plates of black mica.The upper part of this great bed is slightly cellular; the lower part compact: the thickness varied a little in different parts.Manifestly a submarine lava; and is allied to bed 11.
16th and 17th.Dull purplish, calcareous, fine-grained, compact sandstones, which pass into coarse white conglomerates with numerous particles of quartz.
18th.Several alternations of red conglomerate, purplish sandstone, and submarine lava, like that singular rock forming bed 13.
19th.A very heavy, compact, greenish-black stone, with a fine-grained obviously crystalline basis, containing a few specks of white calcareous spar, many specks of the crystallised hydrous red oxide of iron, and some specks of a green mineral; there are veins and nests filled with epidote:
certainly a submarine lava.
20th.Many thin strata of compact, fine-grained, pale purple sandstone.
21st.Gypsum in a nearly pure state, about three hundred feet in thickness:
this bed, in its concretions of anhydrite and layers of small blackish crystals of carbonate of lime, exactly resembles the great gypseous beds in the Peuquenes range.
22nd.Pale purple and reddish sandstone, as in bed 20: about three hundred feet in thickness.
23rd.A thick mass composed of layers, often as thin as paper and convoluted, of pure gypsum with others very impure, of a purplish colour.
24th.Pure gypsum, thick mass.
25th.Red sandstones, of great thickness.
26th.Pure gypsum, of great thickness.
27th.Alternating layers of pure and impure gypsum, of great thickness.
I was not able to ascend to these few last great strata, which compose the neighbouring loftiest pinnacles.The thickness, from the lowest to the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds beneath Iestimated at 3,000 feet, and this does not include either the lower parts of the porphyritic conglomerate, or the altered clay-slate; I conceive the total thickness must be about six thousand feet.I distinctly observed that not only the gypsum, but the alternating sandstones and conglomerates were lens-shaped, and repeatedly thinned out and replaced each other: thus in the distance of about a mile, a bed 300 feet thick of sandstone between two beds of gypsum, thinned out to nothing and disappeared.The lower part of this section differs remarkably,--in the much greater diversity of its mineralogical composition,--in the abundance of calcareous matter,--in the greater coarseness of some of the conglomerates,--and in the numerous particles and well-rounded pebbles, sometimes of large size, of quartz,--from any other section hitherto described in Chile.From these peculiarities and from the lens-form of the strata, it is probable that this great pile of strata was accumulated on a shallow and very uneven bottom, near some pre-existing land formed of various porphyries and quartz-rock.The formation of porphyritic claystone conglomerate does not in this section attain nearly its ordinary thickness; this may be PARTLYattributed to the metamorphic action having been here much less energetic than usual, though the lower beds have been affected to a certain degree.
If it had been as energetic as in most other parts of Chile, many of the beds of sandstone and conglomerate, containing rounded masses of porphyry, would doubtless have been converted into porphyritic conglomerate; and these would have alternated with, and even blended into, crystalline and porphyritic strata without a trace of mechanical structure,--namely, into those which, in the present state of the section, we see are unquestionably submarine lavas.
The beds of gypsum, together with the red alternating sandstones and conglomerates, present so perfect and curious a resemblance with those seen in our former section in the basin-valley of Yeso, that I cannot doubt the identity of the two formations: I may add, that a little westward of the P.
del Inca, a mass of gypsum passed into a fine-grained, hard, brown sandstone, which contained some layers of black, calcareous, compact, shaly rock, precisely like that seen in such vast masses on the Peuquenes range.
Near the Puente del Inca, numerous fragments of limestone, containing some fossil remains, were scattered on the ground: these fragments so perfectly resemble the limestone of bed No.3, in which I saw impressions of shells, that I have no doubt they have fallen from it.The yellow magnesian limestone of bed No.10, which also includes traces of shells, has a different appearance.These fossils (as named by M.d'Orbigny) consist of:--
Gryphaea, near to G.Couloni (Neocomian formation).
Arca, perhaps A.Gabrielis, d'Orbigny, "Pal.Franc." (Neocomian formation).
Mr.Pentland made a collection of shells from this same spot, and Von Buch considers them as consisting of:--Trigonia, resembling in form T.costata.
Pholadomya, like one found by M.Dufresnoy near Alencon.
Isocardi excentrica, Voltz., identical with that from the Jura.
