444 Tierra del Fuego. part n. 



thinned out to half its thickness, and had one of its 

 walls jagged, with fragments of the slate embedded 

 in it. 



In southern T. del Fuego, the clay- slate towards 

 its SW. boundary, becomes much altered and feld- 

 spathic. Thus on "Wollaston Island slate and grauwacke 

 can be distinctly traced passing into feldspathic rocks 

 and greenstones, including iron pyrites and epidote, 

 but still retaining traces of cleavage with the usual 

 strike and dip. One such metamorphosed mass was 

 traversed by large vein-like masses of a beautiful 

 mixture (as ascertained by Professor Miller) of green 

 epidote, garnets, and white calcareous spar. On the 

 northern point of this same island, there were various 

 ancient submarine volcanic rocks, consisting of amyg- 

 daloids with dark bole and agate, — of basalt with 

 decomposed oliviue, — of compact lava with glassy 

 feldspar, — and of a coarse conglomerate of red scoriae, 

 parts being amygdaloidal with carbonate of lime. The 

 southern part of Wollaston Island and the whole of 

 Hermite and Horn Islands, seem formed of cones of 

 greenstone : the outlying islets of II Defenso and D. 

 Eaminez are said l to consist of porphyritic lava. In 

 crossing Hardy Peninsula, the slate still retaining 

 traces of its usual cleavage, passes into columnar feld- 

 spathic rocks, which are succeeded by an irregular tract 

 of trappean and basaltic rocks, containing glassy feld- 

 spar and much iron pyrites : there is, also, some harsh 

 red claystone porphyry, and an almost true trachyte, 

 with needles of hornblend, and in one spot a curious 

 slaty rock divided into quadrangular columns, having 

 a base almost like trachyte, with drusy cavities lined 

 by crystals, too imperfect, according to Professor Miller, 



1 Determined by Professor Jameson. Weddell's * Voyage,' p. 1C9. 



