PEOPYLITE AKD 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 

 t} 7 pes 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. 



Group III.— SUB -BASIC BOCKS— PEOPYLITE AND ANDESITE. 



Sub-groups. 



Characteristics. 



1. HOKNBLENDIC PEOPYLITE .. 



Consistiug of predominant plagioclaso and subordinate orthoclase, the 

 former especially, in large, well-formed crystals, abundantly dissem- 

 inated throughout a compact, homogeneous base. The fracture is 

 superficially like diorite or other medium-grained granitoid rocks. 

 The varieties usually are olive or tawny green color, sometimes red- 

 dish, or the green and red are banded, the former greatly predominat- 

 ing. Hornblende is rarely conspicuous to the eye, but in the micro- 

 scope is seen in abundance in small fragments, disseminated dust- 

 like, or in spangles. It is pale green and with sharply-defined edges. 

 Biotite and brown hornblende sparingly occur. The facies of the rock 

 suggests that it has been more or less altered and the microscope and 

 chemical analysis confirm it. 



2. Augitic propylite (?) 



This rock is mentioned by Richthofen, but has not been recognized in 

 the High Plateaus. 



3. Quartz propylite 



A rock having the essential characters of hornblendic propylite, but 

 with the addition of a notable amount of free quartz. It is generally 

 a more siliceous rock than the latter and in most occurrences is 

 fresher in appearance. 



