In descending the eastern slope of this great central range, the strata, which in the upper part dip eastward at about an angle of 40 degrees, become more and more curved, till they are nearly vertical; and a little further onwards there is seen on the further side of a ravine, a thick mass of strata of bright red sandstone [T], with their upper extremities slightly curved, showing that they were once conformably prolonged over the beds [S]: on the southern and opposite side of the road, this red sandstone and the underlying black shaly rocks stand vertical, and in actual juxtaposition.Continuing to descend, we come to a synclinal valley filled with rubbish, beyond which we have the red sandstone [T2] corresponding with [T], and now dipping, as is seen both north and south of the road, at 45 degrees to the west; and under it, the beds [S2, R2, Q2, and I believe P2] in corresponding order and of similar composition, with those on the western flank of the Pequenes range, but dipping westward.Close to the synclinal valley the dip of these strata is 45 degrees, but at the eastern or farther end of the series it increases to 60 degrees.Here the great gypseous formation abruptly terminates, and is succeeded eastward by a pile of more modern strata.Considering how violently these central ranges have been dislocated, and how very numerous dikes are in the exterior and lower parts of the Cordillera, it is remarkable that I did not here notice a single dike.The prevailing rock in this neighbourhood is the black, calcareous, compact shale, whilst in the valley-basin of the Yeso the purplish red sandstone or mudstone predominates,--both being associated with gypseous strata of exactly the same nature.It would be very difficult to ascertain the relative superposition of these several masses, for we shall afterwards see in the Cumbre Pass that the gypseous and intercalated beds are lens-shaped, and that they thin out, even where very thick, and disappear in short horizontal distances: it is quite possible that the black shales and red sandstones may be contemporaneous, but it is more probable that the former compose the uppermost parts of the series.
The fossils above alluded to in the black calcareous shales are few in number, and are in an imperfect condition; they consist, as named for me by M.d'Orbigny, of:--1.Ammonite, indeterminable, near to A.recticostatus, d'Orbigny, "Pal.
Franc." (Neocomian formation).
2.Gryphaea, near to G.Couloni (Neocomian formations of France and Neufchatel).
3.Natica, indeterminable.
4.Cyprina rostrata, d'Orbigny, "Pal.Franc." (Neocomian formation).
5.Rostellaria angulosa (?), d'Orbigny, "Pal.de l'Amer.Mer."6.Terebratula (?).
Some of the fragments of Ammonites were as thick as a man's arm: the Gryphaea is much the most abundant shell.These fossils M.d'Orbigny considers as belonging to the Neocomian stage of the Cretaceous system.Dr.
Meyen, who ascended the valley of the Rio Volcan, a branch of the Yeso, found a nearly similar, but apparently more calcareous formation, with much gypsum, and no doubt the equivalent of that here described ("Reise um Erde"etc.Th.1 s.355.): the beds were vertical, and were prolonged up to the limits of perpetual snow; at the height of 9,000 feet above the sea, they abounded with fossils, consisting, according to Von Buch ("Descript.Phys.
des Iles Canaries" page 471.), of:--
1.Exogyra (Gryphaea) Couloni, absolutely identical with specimens from the Jura and South of France.
2.Trigonia costata, identical with those found in the upper Jurassic beds at Hildesheim.
3.Pecten striatus, identical with those found in the upper Jurassic beds at Hildesheim.
4.Cucullaea, corresponding in form to C.longirostris, so frequent in the upper Jurassic beds of Westphalia.
5.Ammonites resembling A.biplex.
Von Buch concludes that this formation is intermediate between the limestone of the Jura and the chalk, and that it is analogous with the uppermost Jurassic beds forming the plains of Switzerland.Hence M.
D'Orbigny and Von Buch, under different terms, compare these fossils to those from the same late stage in the secondary formations of Europe.
Some of the fossils which I collected were found a good way down the western slope of the main ridge, and hence must originally have been covered up by a great thickness of the black shaly rock, independently of the now denuded, thick, overlying masses of red sandstone.I neglected at the time to estimate how many hundred or rather thousand feet thick the superincumbent strata must have been: and I will not now attempt to do so.
This, however, would have been a highly interesting point, as indicative of a great amount of subsidence, of which we shall hereafter find in other parts of the Cordillera analogous evidence during this same period.The altitude of the Peuquenes Range, considering its not great antiquity, is very remarkable; many of the fossils were embedded at the height of 13,210feet, and the same beds are prolonged up to at least from fourteen to fifteen thousand feet above the level of the sea.
THE PORTILLO OR EASTERN CHAIN.