472 COSMOS. 



the seemingly amorphous mass to be analysed. The result 

 is obtained in a more convincing and more certain manner 

 if the principal mass, as well as the chief elements of the 

 mixture, can be singly investigated both mineralogically and 

 chemically. This is the case with the trachytes of the Peak 

 of Teneriffe and those of Etna. The supposition that the 

 principal mass consists of the same small, inseparable, com- 

 ponent parts which we recognise in the large crystals appears 

 to be by no means well grounded, for, as we have already 

 noticed, as shown in Charles Deville's work, the apparently 

 amorphous principal mass generally furnishes more silicic 

 acid than would be expected from the nature of the felspar 

 and the other visible commixed elements. Among the 

 leucite-ophyrs, as Gustav Rose observes, a striking contrast is 

 exhibited, even in the specific difference of the prevailing 

 alkalies (of the potash containing interspersed leucites) and 

 the almost exclusively natroniferous principal mass. 87 



But along with these associations of augite with oligoclaee, 

 augite with labradorite, and hornblende with oligoclase, which 

 are referred to in our classification of the trachytes, and which 

 especially characterise them, there exist likewise in each vol- 



whole very unusual in basalt, and yet some of the basaltic summits of 

 the Bohemian central mountains, first described by Reuss, Freieslebeu, 

 and myself, contain plenty of it. The unusual isolation of certain 

 mineral bodies, and the causes of their legitimate specific association, 

 probably depend on many still undiscovered causes of pressure, tempe- 

 rature, fluidity, and rapidity in cooling. The specific differences of the 

 .association are, however, of great importance, both in the mixed rocks 

 and in the masses of mineral veins ; and in geological descriptions, noted 

 down in the open air, in sight of the object described, the observer 

 should be careful not to make any mistake as to what may be a prevail- 

 ing, or at least a rarely absent member of the association, and what 

 may be sparingly or only accidentally combined. The diversity which 

 prevails in the elements of a mixture, for instance, in the trachytes, 

 is repeated, as I have already noticed, in the rocks themselves. In both 

 continents there exist large tracts of country in which trachyte forma- 

 tions and basalt formations as it were repel each other, as basalts and 

 phonolites; and there are other countries in which trachytes and 

 basalts alternate with each other in tolerably close proximity (see 

 Gustav Jenzsch, MonograpJiie der bohmischen Pkonolithe, 1856, s. 

 17). 



s " See Bischof, Chemische und pkysikalische Geologic, Bd. ii, 

 185], & 2288, 2297; Roth, Monographic des Vesuvs. 1857, s, 

 805. 



