第77章 ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND
Cruz, and, according to M.d'Orbigny, 120 miles north of St.Fe.At S.Cruz we have seen that it extends across the continent; being on the coast about eight hundred feet in thickness (and rather more at S.Julian), and rising with the contemporaneous lava-streams to a height of about three thousand feet at the base of the Cordillera.It rests, wherever any underlying formation can be seen, on plutonic and metamorphic rocks.Including the newer Pampean deposit, and those strata in Eastern Tierra del Fuego of doubtful age, as well as the boulder formation, we have a line of more than twenty-seven degrees of latitude, equal to that from the Straits of Gibraltar to the south of Iceland, continuously composed of tertiary formations.Throughout this great space the land has been upraised, without the strata having been in a single instance, as far as my means of observation went, unequally tilted or dislocated by a fault.
TERTIARY FORMATIONS ON THE WEST COAST.
CHONOS ARCHIPELAGO.
The numerous islands of this group, with the exception of Lemus, Ypun, consist of metamorphic schists; these two islands are formed of softish grey and brown, fusible, often laminated sandstones, containing a few pebbles, fragments of black lignite, and numerous mammillated concretions of hard calcareous sandstone.Out of these concretions at Ypun (latitude 40degrees 30 minutes S.), I extracted the four following extinct species of shells:--1.Turritella suturalis, G.B.Sowerby (also Navidad).
2.Sigaretus subglobosus, G.B.Sowerby (also Navidad).
3.Cytheraea (?) sulculosa (?), G.B.Sowerby (also Chiloe and Huafo?).
4.Voluta, fragments of.
In the northern parts of this group there are some cliffs of gravel and of the boulder formation.In the southern part (at P.Andres in Tres Montes), there is a volcanic formation, probably of tertiary origin.The lavas attain a thickness of from two to three hundred feet; they are extremely variable in colour and nature, being compact, or brecciated, or cellular, or amygdaloidal with zeolite, agate and bole, or porphyritic with glassy albitic feldspar.There is also much imperfect rubbly pitchstone, with the interstices charged with powdery carbonate of lime apparently of contemporaneous origin.These lavas are conformably associated with strata of breccia and of brown tuff containing lignite.The whole mass has been broken up and tilted at an angle of 45 degrees, by a series of great volcanic dikes, one of which was thirty yards in breadth.This volcanic formation resembles one, presently to be described, in Chiloe.
HUAFO.
This island lies between the Chonos and Chiloe groups: it is about eight hundred feet high, and perhaps has a nucleus of metamorphic rocks.The strata which I examined consisted of fine-grained muddy sandstones, with fragments of lignite and concretions of calcareous sandstone.I collected the following extinct shells, of which the Turritella was in great numbers:--1.Bulla cosmophila, G.B.Sowerby.
2.Pleurotoma subaequalis, G.B.Sowerby.
3.Fusus cleryanus, d'Orbigny, "Voyage Pal." (also at Coquimbo).
4.Triton leucostomoides, G.B.Sowerby.
5.Turritella Chilensis, G.B.Sowerby (also Mocha).
6.Venus, probably a distinct species, but very imperfect.
7.Cytheraea (?) sulculosa (?), probably a distinct species, but very imperfect.
8.Dentalium majus, G.B.Sowerby.
CHILOE.
This fine island is about one hundred miles in length.The entire southern part, and the whole western coast, consists of mica-schist, which likewise is seen in the ravines of the interior.The central mountains rise to a height of 3,000 feet, and are said to be partly formed of granite and greenstone: there are two small volcanic districts.The eastern coast, and large parts of the northern extremity of the island are composed of gravel, the boulder formation, and underlying horizontal strata.The latter are well displayed for twenty miles north and south of Castro; they vary in character from common sandstone to fine-grained, laminated mudstones: all the specimens which I examined are easily fusible, and some of the beds might be called volcanic grit-stones.These latter strata are perhaps related to a mass of columnar trachyte which occurs behind Castro.The sandstone occasionally includes pebbles, and many fragments and layers of lignite; of the latter, some are apparently formed of wood and others of leaves: one layer on the N.W.side of Lemuy is nearly two feet in thickness.There is also much silicified wood, both common dicotyledonous and coniferous: a section of one specimen in the direction of the medullary rays has, as I am informed by Mr.R.Brown, the discs in a double row placed alternately, and not opposite as in the true Araucaria.I found marine remains only in one spot, in some concretions of hard calcareous sandstone: in several other districts I have observed that organic remains were exclusively confined to such concretions; are we to account for this fact, by the supposition that the shells lived only at these points, or is it not more probable that their remains were preserved only where concretions were formed? The shells here are in a bad state, they consist of:--1.Tellinides (?) oblonga, G.B.Sowerby (a solenella in M.d'Orbigny's opinion).
2.Natica striolata, G.B.Sowerby.
3.Natica (?) pumila, G.B.Sowerby.
4.Cytheraea (?) sulculosa, G.B.Sowerby (also Ypun and Huafo?).