106 



GEOLOGY OF THE HIGH PLATEAUS. 



ing them from it than we encounter in separating extremely horublendic 

 trachytes from andesites. In these trachytes sanidin is the only important 

 mineral, and though plagioclase and honiblende are not uncommon, they 

 are never conspicuous, and never seem to exert any notable effect upon the 

 character or aspect of the rock. 



In seeking for purely descriptive names, it seems to me that the older 

 trachytes will be sufficiently discriminated if we call thc-m simply }iorn- 

 hlendic trachytes. It occasionally happens that the other group requires 

 to be spoken of collectively, and I shall in such cases employ the term 

 sanidin trachytes, rather than coin a new name. But for precision it may 

 be necessary to subdivide them rather more minutely, since these so-called 

 sanidin-trachytes embrace very wide variations of lithological aspect. The 

 time has not yet come to divide the immense trachytic group according to 

 definite and final principles. To accomplish that will require the careful 

 study of an enormous range of materials. Although my own observation 

 is far too limited to encourage the hope of finding a complete and satis- 

 factory arrangement, I am tempted to give provisionally and tentatively a 

 subdivision embodying such a grouping as will embrace the facts within 

 my knowledge. 



Gkotjp II.— trachytes OR SUB-ACID ROCKS. 



SuB-GROur A.— Sanidin Trachytes. 



