ON ITS EXTERIOR. VOLCANOES. 429 



inclined to use the expressions orthoclase-trachytes, 

 labrador-trachytes, oligoclase-trachytes, and thus to 

 include glassy felspar (sanidine), on account of its che- 

 mical composition, in the genus orthoclase (common 

 felspar). The names were well-sounding and simple ; 

 but their simplicity was itself misleading; for, if the 

 term labrador-trachyte would correctly lead us to Etna 

 and Stromboli, that of oligoclase-trachyte, in its im- 

 portant twofold connection with augite and hornblende^ 

 would falsely connect together the widely distributed, 

 very different formations of Chimborazo and the volcano 

 of Toluca. It is the association of a felspathic element 

 with one or two others, which here, as in the case of 

 certain dike formations, presents itself as the charac- 

 teristic indication. 



The following classification of trachytes, distinguished 

 according to the crystals which are found enclosed in 

 them, has been made by Grustav Eose since the winter 

 of 1852. The principal results of this work, in which 

 oligoclase is never confounded with albite, were attained 

 by him 10 years before, when, in his geological examina- 

 tions in the Eiesengebirge, he found that oligoclase was 

 there an essential ingredient in the granite; and his 

 attention being thus directed to the importance of oligo- 

 clase as an essential constituent, he sought for it also in 

 other rocks. ( 594 ) This examination conducted him to 

 the important result that albite is never a constituent of 

 a particular kind of rock. (Poggend. Ann. Bd. Ixvi. 

 1845, S. 109.) 



First Division. "The ground-mass contains only 

 crystals of glassy felspar; these are in the form of 

 tablets, and are usually large. Hornblende and mica 



