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Updated: May 5, 2025
Near the base of this hill, I observed beds of white tuff, intersected by numerous dikes, some of amygdaloidal basalt and others of trachyte; and beds of slaty phonolite with the planes of cleavage directed N.W. and S.E. Parts of this rock, where the crystals were scanty, closely resembled common clay-slate, altered by the contact of a trap-dike.
Tierra del Fuego, clay-slate formation, cretaceous fossils of; cleavage and foliation; form of land. Chonos Archipelago, mica-schists, foliation disturbed by granitic axis; dikes. Chiloe. Concepcion, dikes, successive formation of. Central and Northern Chile. Concluding remarks on cleavage and foliation. Their close analogy and similar origin. Stratification of metamorphic schists.
I was unable to persuade myself that these great parallel masses had been injected, until I found one dike which abruptly thinned out to half its thickness, and had one of its walls jagged, with fragments of the slate embedded in it. In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary, becomes much altered and feldspathic.
The water is charged with carbonic acid and sulphuretted hydrogen. These springs deposit much tufa in the form of spherical balls. 1st. The lowest mass is the altered clay-slate described in the preliminary discussion, and which in this line of section was here first met with.
The white chalkstone glitters from afar; the light grey feldspar assumes a warm flesh tint; the limestone becomes straw color; the crystals of hornblende flash like fire-flies; and the veins of white quartz, running with their nodules of serpentine and chlorite through the dark clay-slate, gleam as do chain lightnings through the clouds.
The abrupt hillocks thus composed, as well as the highly inclined folia of the common varieties of gneiss, strike N.N.E. or a little more easterly, and S.S.W. Clay-slate is occasionally met with, and near the L. del Potrero, there is white marble, rendered fissile from the presence of hornblende, mica, and asbestus; the cleavage of these rocks and their stratification, that is the alternating masses thus composed, strike N.N.E. and S.S.W. like the foliated gneisses, and have an almost vertical dip.
From the character and composition of the pebbles in the old river-bed, amongst which are Cambrian sandstone, quartzite, clay-slate, and white Jurassic limestone, Sir A. Geikie concludes that when the river was flowing, the island must have been connected with the mainland to the east where the parent masses of these pebbles are found. View of the Scuir of Eigg from the east.
Comparing our section of the Uspallata range with that of the Cumbre, we see, with the exception of the underlying clay-slate, and perhaps of the intrusive rocks of the axes, a striking dissimilarity in the strata composing them.
It consists up to its summit, of quartz, generally pure and white, but sometimes reddish, and divided into thick laminae or strata: in one part there is a little glossy clay-slate with a tortuous cleavage. The thick layers of quartz strike in a W. 30 degrees N. line, dipping southerly at an angle of 45 degrees and upwards.
These insensible passages from primitive to transition strata by clay-slate, which becomes carburetted at the same time that it presents a concordant position with mica-slate and gneiss, have also been observed several times in Europe by celebrated geologists.
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