440 cosmos. 



tav Rose has come to the general conclusion that it is very- 

 doubtful whether albite occurs at all among the minerals as 

 a real and essential element of commixture ; consequently, 

 according to the old conception of andesite, this mineral 

 would actually be wanting in the chain of the Andes. 



The mineralogical condition of the trachytes is imperfectly 

 recognized if the porphyritically inclosed crystals can not be 

 separately examined and measured, in which case the inves- 

 tigator must have recourse, to the numerical proportions of 

 the earths, alkalies, and metallic oxyds which the result of 

 the analysis furnishes, as well as to the specific gravity of the 

 seemingly amorphous mass to be analyzed. The result is 

 obtained in a more convincing and more certain manner if 

 the principal mass, as well as the chief elements of the mix- 

 ture, can be singly investigated both mineralogically and 

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

 of Teneriffe and those of JEtna. The supposition that the 

 principal mass consists of the same small, inseparable com- 

 ponent parts which we recognize 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 feldspar 

 and the other visible commixed elements. Among the leu- 

 cite 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.* 



But along with these associations of augite with oligoclase, 

 augite with Labradorite, and hornblende with oligoclase, 



pressure, temperature, 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 geo- 

 logical 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 prevailing, or at least a rarely absent member of 

 the association, and what may be sparingly or only accidentally com- 

 bined. 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 formations 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, Monographic der boh- 

 mischen JPhonolithe, 1856, s. 1-7). 



* See Bischof, Chemische und PhysikaUsche Geologie, bd. ii., 1851, 

 s. 2288, 2297; Eoth, Monographie des Vesuvs, 1857, s. 305. 



