110 GEOLOGY OF THE HIGH PLATEAUS. 



Gkotjp HI.— SUB-BASIC EOCKS— PEOPYLITE AND ANDESITE— Continued. 



Sub-groups. 



Characteristics. 



4. HORNBLENDIC ANDESITE ... 



Consists of plagioclase, either wholly or with subordinate orthoclase 

 and with hornblende ; the latter usually conspicuous ; the crystals 

 imbedded in a base which is usually moderately fine, sometimes a lit- 

 tle coarse. The color is almost always green, from light to very dark. 

 The fracture is peculiar, splintery or conchoidal, radiating from the 

 point of impact. The hornblendes are mostly of the dark-brown 

 variety ; in the thin section with a black, shaded border. The base 

 shows fluidal structure, but not always. 



5. AUGITIC ANDESITE 



Usually a more basic rock than the foregoing ; feldspar almost wholly 

 plagioclase ; augite taking the place of hornblende ; either gray or 

 nearly black in color, never with greenish cast unless much altered ; 

 the more basic varieties merge into the dolerites .and the less basic 

 into the augitic trachytes by transition. Resemblances to dolerito 

 most frequent. 





G. Dacite or quartz ajtde- 



SITE. 



Containing predominant plagioclase feldspar, with free quartz and al- 

 most always abundant hornblende. It has a somewhat rhyolitic tex- 

 ture and habit. Sometimes biotite replaces the hornblende. 



IV. BASALTS. The classification and subdivision of the basalts pre- 

 sent some difficulty. In the basic lavas we have occurrences in which the 

 minerals leucite and nephelin replace wholly or in part the feldspars, and a 

 question arises as to the importance which is to be attached to this substitu- 

 tion. In the other great groups the subdivisions have rested upon texture and 

 general habitus of the sub-groups as well as upon the occurrence of accessory 

 and subordinate minerals in conspicuous quantity. In the acid and sub- 

 acid rocks accessory minerals are relatively in small proportions and varia- 

 tions of texture and habit very strongly pronounced. In the basic rocks 

 the reverse is true — the accessory minerals are more numerous, almost 

 rivaling the primary ones, while the texture, though considerably varied, 

 is far less so than in the acid rocks. These considerations would lead us 

 to rest the subdivisions rather upon a mineralogical basis than upon a tex- 

 tural one. Some authors separate dolerite from the so-called "true basalts" 

 on textural grounds, the former being macroscopically crystalline while the 

 basalts proper exhibit distinct crystals only under the microscope. Even 



