446 Tierra del Fuego. paet ir. 



great range, there is no clay-slate, but only gneiss, mica, 

 and hornblendic slates, resting on great barren bills of 

 time granite, and forming a tract about sixty miles in 

 width. Again, westward of these rocks, the outermost 

 islands are of trappean formation, which, from infor- 

 mation obtained during the voyages of the Adventure 

 and Beagle, 1 seem, together with granite, chiefly to 

 prevail along the western coast as far north as the en- 

 trance of the St. of Magellan: a little more inland, on 

 the eastern side of Clarence Island and S. Desolation, 

 granite, greenstone, mica-slate, and gneiss appear to 

 predominate. I am tempted to believe, that where the 

 clay-slate has been metamorphosed at great depths 

 beneath the surface, gneiss, mica-slate, and other allied 

 rocks have been formed, but where the action has taken 

 place nearer the surface, feldspathic porphyries, green- 

 stones, etc., have resulted, often accompanied by sub- 

 marine volcanic eruptions. 



Only one other rock, met with in both arms of the 

 Beagle Channel, deserves any notice, namely a granulo- 

 crvstalline mixture of white albite, black hornblende 

 (ascertained by measurement of the crystals, and con- 

 firmed by Professor Miller), and more or less of brown 

 mica, but without any quartz. This rock occurs in 

 large masses, closely resembling in external form 

 granite or syenite : in the southern arm of the Channel, 

 one such mass underlies the mica-slate, on which clay- 

 slate was superimposed : this peculiar plutonic rock 

 which, as we have seen, occurs also in Hardy Peninsula, 

 is interesting, from its perfect similarity with that 

 (hereafter often to be referred to under the name of 



1 See the Paper by Capt. King, in the ' Geograph. Journal ; ' also 

 a Letter to Dr. Fitton in ' Geolog. Proa' vol. i. p. 29 ; also some ob- 

 servations by Capt. FitzKoy, 'Voyages,' vol. i. p. 375. I am indebted 

 also to Mr. Lyell for a series of specimens collected by Lieut. 

 Graves. 



