THE EARTH'S ROCK FOUNDATIONS 67 



Thus in the granite group we have first pumice, then obsidian, 

 then granite, and the granites may vary from very fine-grained 

 to very coarse-grained granite, the latter being not infrequently 

 porphyritic. 



Rhyolite pumice is a spongy glass. It is light in color, porous, 

 and, therefore, light in weight. It is found only in the regions 

 where volcanic action has occurred comparatively recently. 

 Rhyolite obsidian also occurs only in the regions of recent volcanic 

 activity. It is a glassy rock which breaks with a conchoidal 

 fracture. It varies greatly in color from a light to so dark a 

 tint that it is almost black. 



The granites consist essentially of quartz and orthoclase 

 feldspar or at least of feldspars that have so large a mixture of 

 the orthoclase as to have its characters predominant. The 

 granites may be fine-grained or coarse-grained. If one constitu- 

 ent is very coarse-grained and the others more finely crystalline, 

 the granite is spoken of as a porphyritic granite. A number of 

 other minerals besides the two essential ones may be present; 

 mica, especially the biotite form, is very often present, hornblende 

 and pyroxene are frequent ingredients, but never dominant. 

 If these darker minerals are present in quantity, the granite, of 

 course, is very dark. Sometimes the quartz crystals are scattered 

 through the granite in rather regular lines and are frequently 

 twinned, making the rock appear like a slab of feldspar with 

 more or less regular lines of angular quartz figures giving the 

 appearance of Arabic writing; such granite is known as peg- 

 matite. 



Granites are very widely distributed especially in the regions 

 where the older rocks are exposed; for a large proportion of these 

 older rocks are of granitic character. They are found con- 

 sequently as the core of mountain systems where the later sedi- 

 mentary rocks have been worn away from the crest of up-arched 

 strata. They occur abundantly in the Laurentian Highlands 

 of Canada, throughout northern Michigan, Wisconsin, Minne- 

 sota, along the Appalachian Mountains, running through New 



