106 



GEOLOGY OP THE HIGH PLATEAUS. 



ing them from it than we encounter in separating extremely hornblendic 

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

 mineral, and though plagioclase and hornblende 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 them simply horn- 

 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. 



Geotjp II.— trachytes oe sub acid eocks. 



Sub-group A. — Saxidix Trachytes. 





Characteristics. 



1. Granitoid trachytes 



Trachytes having a superficial resemblance to granitic rocks ; holding 

 much orthoclasc and less plagioclase, with few other minerals; a 

 very little biotito and hornblende; crystals conspicuous; a some- 

 what porous base, containing little ferritic matter. Usually very 

 light-colored rocks ; seldom dark gray. 



2. PORPHYRITIC TRACHYTE 



A base resembling that of porphyrite, with very conspicuous and per- 

 fect crystals of orthoclasc (usually the turbid or milky variety), often 

 large. The base very fine, compact, and non-vesicular ; more or less 

 ferritic. sometimes showing a feeble aggregate polarization. The 

 groundmass shows none of that coarse, rough texture so common in 

 other trachytes. 



