Page:The birds of Tierra del Fuego - Richard Crawshay.djvu/27

Rh without any quartz. This occurs in large masses, closely resembling in external form granite or syenite, and is interesting from its perfect similarity to andesite which forms the great injected axes of the Cordillera of Chili.

The stratification of the clay-slate is generally very obscure, whereas the cleavage is remarkably well defined, striking in the extreme east of the island W, and E, and even W.S.W, and E.N.E.; over the main portion, including the Darwin Range. W.N.W, and E.S.E.; in the central and western portion of the Strait of Magellan N.W, and S.E.; and north of the Strait nearly N, and S.

Of the islands to the extreme south, it is interesting to note that on Wollaston Island slate and grauwacke can be distinctly traced passing into felspathic rocks and greenstones, including iron pyrites and epidote, but still retaining traces of cleavage with the usual strike and dip. One such metamorphosed mass is transversed by large vein-like masses of a beautiful mixture of green epidote, garnets, and white calcareous spar. On the northern portion of this same island, there are various ancient submarine volcanic rocks, consisting of amygdaloids with dark bole and agate,—of basalt with decomposed olivine,—of compact lava with glassy felspar,—and of a coarse conglomerate of red scoriæ, parts being amygdaloidal with carbonate of lime. The southern part of Wollaston Island and the whole of Hermite and Horn Islands are formed of cones of greenstone.

The external features of Tierra del Fuego are exceedingly varied. There are lowland flats with vast marshes and lakes more or less brackish, scrub-covered downs, bleak black peaty moors, practically impenetrable forests, and regions of everlasting snow probably never trodden by man. In the coastline, there is also much diversity. This may be low shingly beach with the land dead flat behind it, or bare perpendicular cliff washed at foot by the sea, or solid jagged rock overgrown to high-tide mark with impermeable thorn scrub, or else precipitous mountain covered with forest or glacier to the very ocean.