THE GLACIAL PERIOD 



75 



increasing in weight as the plagioclases replace the orthoclase, 

 and the augite, hornblende, and olivine at the end largely 

 replace even the plagioclase. 



Vertically in each column the members of any group are 

 named from distinctly volcanic rocks down to those that are 

 wholly plutonic. Going down the column, the members of the 

 families are less porous, less glassy, the component crystals are 

 constantly larger, and the rocks increase in specific gravity. 



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. 



