THE GLACIAL PERIOD 



75 



The granites contain quartz and orthoclase, often with some 

 hornblende, mica, or augite. (The composition of the gneisses 

 is the same, but they show evidences of stratification see below.) 

 If any one or two minerals are especially conspicuous, the granite 

 is named accordingly, as hornblende granite, biotite-hornblende 

 granite. If quartz is present in twin crystals in a matrix of 

 feldspar, giving an appearance of cuneiform writing on a feld- 

 spar background, the granite is pegmatite. 



Syenite contains an abundance of orthoclase, usually the 

 red varieties and little or no quartz. Hornblende, mica, and 

 augite may any one or all be present. It is a plutonic rock and 

 therefore usually coarse grained. The corresponding volcanic 

 rock is trachyte, finer grained, more porous, and lighter in 

 weight. 



In the diorites the dark feldspars are predominant, though 

 sometimes the lighter-colored plagioclases are abundant. Quartz 



