436 REACTION OF THE INTERIOR OF THE EARTH 



of rock, and from a chain whose age is indicated by the 

 superposition of organic imbedded fossils ; nor is there 

 any objection to names taken from individual moun- 

 tains, or to the use of such expressions as " Teneriffe-" 

 <or " Etna-trachytes " for particular oligoclase or labra- 

 dorite formations. While there was a disposition to 

 recognise everywhere among the very different kinds of 

 felspar belonging to the trachytes of the Andes the 

 presence of albite, every rock in which it was supposed 

 to occur was termed " Andesite." I first find this name 

 with the positive definition, "Andesite is formed by 

 predominating albite and some hornblende," in an im- 

 portant memoir of Leopold von Buch, in 1835, "On 

 Craters of Elevation and Volcanoes." ( 609 ) This dis- 

 position to see albite everywhere lasted for five or six 

 years, until, in mpartially renewed and more thorough 

 examinations, the trachytic albites were recognised as 

 oligoclase. ( 61 ) Grustav Eose has arrived at doubting 

 whether albite presents itself at all in these rocks as an 

 essential ingredient; and thus Andesite, according to 

 the older view of its character, would be wanting in the 

 chain of the Andes itself. 



The mineralogical constitution of trachytes is im- 

 perfectly discerned when the porphyritically imbedded 

 crystals are not capable of being detached from the 

 ground-mass so as to be separately examined and mea- 

 sured, and when it is necessary to have recourse to the 

 numerical proportions of the earths, alkalies, and me- 

 tallic oxides, as given by analysis, and to the specific 

 gravity of the apparently amorphous mass which has to 

 be analysed. The desired result is obtained in a more 

 convincing and certain manner when both the ground- 



