TRUE VOLCANOES. 423 



trachytes, or Labrador trachytes, or oligoclase trachytes, thus 

 comprehending the glassy 'feldspar (sanidinc), on account of 

 its chemical composition, under the species orthoclase (com- 

 mon feldspar). The terms were at least well-sounding and 

 simple; but their very simplicity must have induced error; 

 for, though Labrador trachyte points to ./Etna and to Strom- 

 boli, yet oligoclase trachyte, in its important two-fold com- 

 bination with augite and hornblende, would erroneously con- 

 nect the widely diffused and very dissimilar formations of 

 Chirnborazo and the volcano of Toluca. It is the associa- 

 tion of a feldspathic element with one or two others which 

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

 tion of some mineral dikes. 



The following is a view of the divisions into which Gustav 

 Rose, subsequently to the winter of 1852, distributes the tra- 

 chytes, in reference to the crystals inclosed in them, and 

 separately recognizable. 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. ^ This exam- 

 ination led to the important result (Poggend., Ann., bd. lxvi., 

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

 composition of any rock. 



First Division. — " The principal mass contains only crys- 

 tals of glassy feldspar, 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 

 admixture. To this division belongs the trachyte of the 

 Phlegra^an Fields (Monte Olibano, near Pozzuoli), that of 

 Ischia and of La Tolfa, as also a part of the Mont Dore (the 



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

 dorffs Ann., bd. lvi., 1842, s. 617. Berzelius had found the oligoclase, 

 his "Natron Spodumen," only in a dike 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 fusihility. 

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

 be 2-682 was analyzed by Rammclsberg (Ffandwdrterbuch der Miner- 

 alog., supplcm. i., s. 104: and G. Rose, Ueber die zur Granitgmppe 

 gehurenden Gcbirgsarten, in the Zeitschr. der Deutschen geol. Gesell- 

 schaft. bd. i., 1849, s. 364). 



