PEOPYLITE AND ANDESITE. 



109 



ordinate orthoclase, and with proportions of augite and magnetite very- 

 much smaller than is usual in the basaltic group. We have also vari- 

 eties in which the orthoclase is much less though still notable, and the 

 augite and magnetite, accompanied with glassy or slaggy material included 

 in the groundmass, are very copious; and there are many intermediate 

 varieties. It seems probable that Richthofen may have contemplated only 

 the former in his expression of the characters of augitic andesite, while 

 Zirkel, taking the entire range of variety as one sub-group, with the more 

 augitic and vitreous ones as the type, did not find reasons for separating 

 them, and, therefore, placed them together among the basalts, to which his 

 types certainly most nearly approach. It must be admitted that a hard 

 and fast line cannot be drawn within this range, nor can it be satisfactorily 

 drawn between the more acid augitic andesites and the augitic trachytes. 

 Nevertheless, it seems advisable to draw one arbitrarily, and place the more 

 acid varieties among the andesites and the more basic among the basalts 

 (dolerite), thus following Richthofen rather than Zirkel. 



Geoup UI.— SUB-BASIO EOCKS— PEOPYLITE AND ANDESITE. 



