chap. xiii. Cleavage of Clay -Slate. 445 



to be measured, but resembling Zeagonite. 1 In the 

 midst of these singular rocks, no doubt of ancient sub- 

 marine volcanic origin, a high hill of feldspathic clay- 

 slate projected, retaining its usual cleavage. Near this 

 point, there was a small hillock, having the aspect of 

 granite, but formed of white albite, brilliant crystals of 

 hornblende (both ascertained by the reflecting gonio- 

 meter) and mica; but with no quartz. No recent 

 volcanic district has been observed in any part of Tierra 

 del Fuego. 



Five miles west of the bifurcation of the Beagle 

 Channel, the slate-formation, instead of becoming, as 

 in the more southern parts of Tierra del Fuego, feld- 

 spathic, and associated with trappean or old volcanic 

 rocks, passes by alternations 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-laminae of the slate are straight. These 

 schists compose the chief mountain chain of southern 

 T. del Fuego, ranging along the north side of the north- 

 ern arm of the Beagle Channel, in a short WNW. 

 and ESE. line, with two points (Mounts Sarmiento and 

 Darwin) rising to heights of 6,800 and 6,900 feet. 

 On the south-western side of this northern arm of the 

 Beagle Channel, the clay-slate is seen with its strata 

 dipping from the great chain, so that the metamorphic 

 schists here form a ridge bordered on each side by clay- 

 slate. Farther north, however, to the west of this 



1 See Mn Brooke's Paper in the 'London Phil. Mag.' vol. x. 

 This mineral occurs in an ancient volcanic rock near Rome. 



