ON ITS EXTERIOR, VOLCANOES. 437 



mass itself and its contents can be separately examined, 

 both oryctognostically and chemically. This is the case, 

 for example, in the trachytes of the Peak of TenerifTe, 

 and in those of Etna. The assumption that the ground- 

 mass consists of the same minute undistinguishable 

 constituents which we recognise in the large crystals, 

 appears by no means solidly established ; for, as we have 

 already seen above, in Charles Deville's ingeniously exe- 

 cuted investigations, the apparently amorphous ground- 

 masses for the most part furnish more silicic acid than 

 would be expected from the felspar and other visible 

 ingredients. In leucite-ophyrs, as Grustav Eose remarks, 

 a striking contrast shows itself in the specific differences 

 of the prevailing alkalies, i. e. those of the inwoven 

 potassa-containing leucites, and of the ground-mass 

 which contains almost solely soda.( 611 ) 



But besides these associations of augite with oligo- 

 clase, augite with labradorite, and hornblende with 

 oligoclase, which have been instanced in the above 

 assumed classification of trachytes, and by which they 

 are specially characterised, we find in each volcano 

 other easily recognisable non-essential ingredients, whose 

 frequency or constant absence in different but often 

 neighbouring volcanoes is remarkable. It is probable 

 that this fact of the frequent occurrence of a particular 

 ingredient, or of its having only appeared at widely 

 separated intervals of time, has depended on a variety 

 of conditions, such as the depth at which the substances 

 have originated, temperature, pressure, degree of fluidity, 

 and slow or rapid cooling. The specific association or 

 absence of particular ingredients is opposed to certain 

 theories ; for example, to the supposed origin of pumice 



