TRUE VOLCANOES. 



463 



a similar fragment, with distinct uralite crystals, from tlie 

 slope of the Tunguragua at an elevation of 13,260 feet. 

 Gustav Hose considers this specimen strikingly different 



uncouibined silicic acid, and if we were to suppose labradorite in the 

 rock, a greater quantity of silicic acid would remain over." 



A careful comparison of several analyses for which I am indebted 

 to the friendship of M. Charles Sainte-Claire Deville, to whom the 

 valuable geological collections of our mutual friend Boussingault are 

 accessible for chemical experiment, shows that the quantity of silicic 

 acid contained in the fundamental mass of the trachytic rocks is gene- 

 rally greater than in the felspars which they contain. The table kindly 

 communicated to me by the compiler himself in the month of June, 

 1857, contains only five of the great volcanoes of the chain of the 

 Andes : ' 



" These differences, as far as regards the relative richness in silica of 

 the ground-mass (and the felspar)," continues Charles Deville, " will 

 appear still more striking when it is considered that, in analysing a, 

 rock en masse, there are included in the analysis, along with the basis 

 properly so called, not only fragments of felspar similar to those which 

 have been extracted, but even such minerals as amphibole, pyroxene, 

 and especially peridote, which are less rich in silica than the felspar. 

 This excess of silica manifests itself sometimes by the presence of iso- 

 lated grains of quartz, which M. Abich has detected in the trachytes 

 of the Drachenfels (Siebengebirge, near Bonn), and which I have myself 

 observed with some surprise in the trachytic dolerite of Guadaloupe." 



" If," observes Gustav Rose, " we add to this remarkable synopsis of 

 the silicic acid contained in Chimborazo the result of the latest analysis, 



