Page:The birds of Tierra del Fuego - Richard Crawshay.djvu/23

Rh ears, and His work bein,^ carried on under his eyes, he cannot but live always as much in realization of things unseen as of things seen:—

The geology of Tierra del Fuego is an impressive study. According to Darwin, than whom no better authority has arisen in all this time, the northern and eastern portion is composed of horizontal tertiary strata, fringed by low, irregular, extensive plains belonging to the boulder formation, and made up of coarse unstratified masses, sometimes associated with fine, laminated, muddy sandstones. Alluvial gold occurs freely; also lignitic coal at Cheena Creek and on the south coast. In San Sebastian Bay, the cliffs are composed of fine sandstones often in curvilinear layers, including hard concretions of calcareous sandstone, and layers of gravel. Towards the interior of the island, the tertiary formation is bounded by a broad mountainous band of clay slate. The rock forming the summit of Nose Peak is determined by Dr. J. S. Flett as a fine, highly felspathic grit, yellow in colour when weathered and soft from the abundance of felspar. It consists of quartz, various felspar, fragments of slate or shale, biotite decomposing to chlorite, and a minutely fragmental interstitial or cementing material. Metamorphic schists, granite, and various trappean rocks compose the western and broken portion. No recent volcanic district occurs anywhere. The clay slate is generally fissile, sometimes siliceous or ferruginous with veins of quartz and calcareous spar; often assuming, especially on the loftier mountains, an altered felspathic character, passing into felspathic porphyry;