Page:The birds of Tierra del Fuego - Richard Crawshay.djvu/24

xii occasionally associated with breccia and grauwacke. At Good Success Bay, there is a little intercalcated black crystalline limestone. In many parts the clay slate is broken by dikes and by great masses of greenstone, often highly hornblendic. Near the dikes the slate generally becomes paler-coloured, harder, less fissile, of a felspathic nature, and passes into a porphyry or greenstone: in one case, however, it becomes more fissile, of a red colour, and contains minute scales of mica, which are absent in the unaltered rock. Towards its south-west boundary, the clay-slate becomes much altered and felspathic. West of the bifurcation of the Beagle Channel, the slate-formation, instead of becoming, as in the more southern parts of the island, felspathic, and associated with trappean or old volcanic rocks, passes by alterations into a great underlying mass of fine gneiss and glossy clay-slate, which at no great distance is succeeded by a grand formation of mica-slate containing garnets. The folia of these metamorphic schists strike parallel to the cleavage-planes of the clay-slate, which have a very uniform direction over the whole of this part of the country: the folia, however, are undulatory and tortuous, whilst the cleavage-laminæ of the slate are straight. The Darwin Range is composed of these schists. On the south-western side of the northern arm of the Beagle Channel, the clay-slate has its strata dipping from this great mountain chain, so that the metamorphic schists here form a ridge bordered on each side by clay«slate. Further north, to the west of this great range there is no clay-slate, but only gneiss, mica, and hornblendic slates, resting on great barren hills of true granite, and forming a tract about sixty miles in Width. Westward of these rocks, the outermost islands are of trappean formation, which, together with granite, seem chiefly to prevail along the western coast as far north as the entrance to the Strait of Magellan. In both arms of the Beagle Channel, there is a peculiar plutonic rock deserving of especial notice, namely, a granulo-crystalline mixture of White albite, black hornblende, and more or less of brown mice, but