第79章 ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND

The various tufaceous and other beds at this northern end of Chiloe probably belong to about the same age with those near Castro, and they contain, as there, many fragments of black lignite and of silicified and pyritous wood, often embedded close together.They also contain many and singular concretions: some are of hard calcareous sandstone, in which it would appear that broken volcanic crystals and scales of mica have been better preserved (as in the case of the organic remains near Castro) than in the surrounding mass.Other concretions in the white brecciola are of a hard, ferruginous, yet fusible, nature; they are as round as cannon-balls, and vary from two or three inches to two feet in diameter; their insides generally consist either of fine, scarcely coherent volcanic sand (The frequent tendency in iron to form hollow concretions or shell containing incoherent matter is singular; D'Aubuisson ("Traite de Geogn." tome 1 page 318) remarks on this circumstance.), or of an argillaceous tuff; in this latter case, the external crust was quite thin and hard.Some of these spherical balls were encircled in the line of their equators, by a necklace-like row of smaller concretions.Again there were other concretions, irregularly formed, and composed of a hard, compact, ash-coloured stone, with an almost porcelainous fracture, adhesive to the tongue, and without any calcareous matter.These beds are, also, interlaced by many veins, containing gypsum, ferruginous matter, calcareous spar, and agate.It was here seen with remarkable distinctness, how intimately concretionary action and the production of fissures and veins are related together.Figure 20 is an accurate representation of a horizontal space of tuff, about four feet long by two and a half in width: the double lines represent the fissures partially filled with oxide of iron and agate: the curvilinear lines show the course of the innumerable, concentric, concretionary zones of different shades of colour and of coarseness in the particles of tuff.The symmetry and complexity of the arrangement gave the surface an elegant appearance.It may be seen how obviously the fissures determine (or have been determined by) the shape, sometimes of the whole concretion, and sometimes only of its central parts.The fissures also determine the curvatures of the long undulating zones of concretionary action.From the varying composition of the veins and concretions, the amount of chemical action which the mass has undergone is surprisingly great; and it would likewise appear from the difference in size in the particles of the concretionary zones, that the mass, also, has been subjected to internal mechanical movements.

In the peninsula of Lacuy, the strata over a width of four miles have been upheaved by three distinct, and some other indistinct, lines of elevation, ranging within a point of north and south.One line, about two hundred feet in height, is regularly anticlinal, with the strata dipping away on both sides, at an angle of 15 degrees, from a central "valley of elevation,"about three hundred yards in width.A second narrow steep ridge, only sixty feet high, is uniclinal, the strata throughout dipping westward; those on both flanks being inclined at an angle of from ten to fifteen degrees;whilst those on the ridge dip in the same direction at an angle of between thirty and forty degrees.This ridge, traced northwards, dies away; and the beds at its terminal point, instead of dipping westward, are inclined 12degrees to the north.This case interested me, as being the first in which I found in South America, formations perhaps of tertiary origin, broken by lines of elevation.

VALDIVIA: ISLAND OF MOCHA.

The formations of Chiloe seem to extend with nearly the same character to Valdivia, and for some leagues northward of it: the underlying rocks are micaceous schists, and are covered up with sandstone and other sedimentary beds, including, as I was assured, in many places layers of lignite.I did not land on Mocha (latitude 38 degrees 20 minutes), but Mr.Stokes brought me specimens of the grey, fine-grained, slightly calcareous sandstone, precisely like that of Huafo, containing lignite and numerous Turritellae.

The island is flat topped, 1,240 feet in height, and appears like an outlier of the sedimentary beds on the mainland.The few shells collected consist of:--1.Turritella Chilensis, G.B.Sowerby (also at Huafo).

2.Fusus, very imperfect, somewhat resembling F.subreflexus of Navidad, but probably different.

3.Venus, fragments of.

CONCEPCION.

Sailing northward from Valdivia, the coast-cliffs are seen, first to assume near the R.Tolten, and thence for 150 miles northward, to be continued with the same mineralogical characters, immediately to be described at Concepcion.I heard in many places of beds of lignite, some of it fine and glossy, and likewise of silicified wood; near the Tolten the cliffs are low, but they soon rise in height; and the horizontal strata are prolonged, with a nearly level surface, until coming to a more lofty tract between points Rumena and Lavapie.Here the beds have been broken up by at least eight or nine parallel lines of elevation, ranging E.or E.N.E.and W.or W.S.W.These lines can be followed with the eye many miles into the interior; they are all uniclinal, the strata in each dipping to a point between S.and S.S.E.with an inclination in the central lines of about forty degrees, and in the outer ones of under twenty degrees.This band of symmetrically troubled country is about eight miles in width.