TRUE VOLCANOES. ' 453 



chyles, thus comprehending the glassy felspar (sanidine), on 

 account of its chemical composition, under the species ortho- 

 clase (common felspar). The terms were at least well-sound- 

 ing and simple, but their very simplicity must have induced 

 error, for though labrador-trachyte points to Etna and to 

 Stromboli, yet oligoclase-trachyte, in its important twofold 

 combination with augite and hornblende, would erroneously 

 connect the widely diffused and very dissimilar formations 

 of Chimborazo and the volcano of Toluca. It is the asso- 

 ciation of a felspathic element with one or two others which 

 here forms the characteristic feature, as it does in the forma- 

 tion of some mineral-dykes. 



The following is a view of the divisions into which Gustav 

 Rose, subsequently to the winter of 1852, distributes the 

 trachytes, in reference to the crystals enclosed in them, and 

 separately recognisable. The chief results of this work, in 

 which there is no confounding of oligoclase with albite, were 

 obtained ten years earlier ; when my friend discovered, in 

 the course of his geognostic investigations in the Riesenge- 

 birge, that the oligoclase there formed an essential ingredient 

 of the granite, and his attention being thus directed to the 

 importance of oligoclase as an ingredient of that rock, he 

 was induced to look for it likewise in other rocks. 70 This 

 examination led to the important result (Poggend. Ann. 

 Bd. Ixvi, 1845, s. 109) that albite never forms a part in the 

 mixed composition of any rock. 



First division. " The principal mass contains only crystals 

 of glassy felspar, which are laminar, and in general large. 

 Hornblende and mica either do not occur in it at all, or in 

 extremely small quantity, and as an entirely unessential ad- 

 mixture. To this division belongs the trachyte of the Phle- 

 " See Gustav Rose on the granite of the Riesengebirge, in Poggen- 

 dorff's Ann. Bd. Ivi, 1842, s. 617. Berzelius had found the oligoclase, 

 his " Natron Spodumen," only in a dyke of granite ; in the treatise just 

 cited it is for the first time spoken of as an ingredient in the composi- 

 tion of granite (the mineral itself). Gustav Rose here determined the 

 oligoclase according to its specific gravity, the greater proportion of 

 lime contained in it as compared with albite, and its greater fusibility. 

 The same compound with which he had found the specific gravity to 

 be 2.682 was analysed by Rammelsberg (Handworterbuch der Mineralog. 

 eupplem. i, s. 104, and G. Rose Ueber die zur Granitgruppe gehorenden 

 Gebirgsarten, in the Zeitschr. der Deutschen geol. Gesellschajt, Bd. i, 1849, 

 B. 364). 



