Xll PREFACE 



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. ISTear 

 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, fel- 

 spathic, 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- 

 laminge 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 mica, but 



