M. V^oN BucH on Gabbro. 49 



Kielvlg almost resemble mica slate ; they dip rapidly towards 

 the N.W., that is, towards the interior of the island, and 

 thus serve as a base to the other rocks of which the island 

 is composed. A fine grained granite is observed upon the 

 heights, resting upon the schist, — this granite contains some 

 scattered plates of black mica, with an abundance of horn- 

 blende. Diallage shortly appears as a constituent part of 

 the granite, and in a few more distant rocks the granite is 

 changed into a fine grained gabbro. Further still, towards 

 the centre of the island, gabbro forms masses of 1400 feet in 

 height. It is very large grained, and altogether resembles 

 that of Zobtenbcrg and Prato. The grey diallage has a 

 laminated structure and is very shining, and a little con- 

 choidal in the cross fracture ; the crystals are often found 

 almost perfect, and in the form of a four-sided prism termi- 

 nated by a four-sided pyramid. The diallage resists de- 

 composition better than the felspar, and forms salient 

 crystals on the surface of the blocks. The extreme rocks of 

 the North Cape are formed of gneiss in very thin lamina;, 

 apparently subordinate to mica slate, the rock generally ob- 

 servable on these coasts. 



It will be observed that gabbro rests on primitive schist 

 in the North. We have seen it at Genoa under transition 

 slate ; its geological place appears therefore to be well de- 

 termined. 



It seems also that gabbro occurs with serpentine in Nor- 

 way, which accompanies it in other situations. This cir- 

 cumstance appears to M. von Buch to be easily explained; 

 he considers serpentine, as we have seen, to be a mixture 

 of different minerals undeterminable from their small size. 

 • M. de Humboldt observed gabbro near Guancavelica, 

 above the Havannah, in the Interior of Cuba, as also widely 

 extended masses of serpentine containing a great quantity of 

 Schiller spar (Diallage metallo'ide). 



Gabbro ought therefore to be considered as a rock widely 

 spread over the surface of the earth, which follows next, in 

 the series of formations, to primitive schist, and is anterior 

 to porphyry. The gabbro formation is intimately connectecJ 



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