438 EEACTION OF THE INTEEIOK OF THE EAHTII 



from glassy felspar or from obsidian. These considera- 

 tions which are by no means peculiar to very recent 

 times, having been touched upon towards the close of 

 the 18th century in reference to the comparison of the 

 trachytes of Hungary with those of Teneriffe engaged 

 my earnest attention for several years, as appears from 

 my journals kept in Mexico and among the Andes. 

 Since that period great advances have been made in 

 lithology, and the less perfect determinations of mineral 

 species which I was then able to make have been either 

 corrected, or established on more secure foundations, by 

 Gustav Kose's long-continued examination and discus- 

 sion of my collections. 



Mica. 



Black or dark-green magnesia-mica is very frequent 

 in the trachytes of Cotopaxi, at a height of 14,470 feet, 

 between Suniguaicu and Quelendana, as well as in the 

 subterranean pumice-stone quarries of Guapulo and 

 Zumbalica, 16 miles from the foot of the volcano. ( 612 ) 

 The trachytes of the volcano of Toluca are also rich in 

 magnesia-mica, which is wanting at Chimb orazo. ( 613 ) 

 In our continent, micas have presented themselves in 

 abundance : at Vesuvius (for example, in the eruptions 

 of 1821 1823, according to Monticelli and Covelli) ; 

 in the Eifel in the ancient volcanic bombs of the 

 Lacher See( 614 ); in the basalt of Meronitz, of the 

 marly Kausawer-Berg, and especially of the Gamayer- 

 Kuppe ( 615 ) in the Bohemian Mittelgebirge ; and, more 

 rarely in phonolite ( 616 ), as in the dolerite of the Kai- 

 serstuhl, near Freiburg. It is a remarkable circum- 

 stance, that in the trachytes and lavas of both continents 



