408 Introduction to the Study of Science 



much as blocks of wood are, although the saws move much 

 more slowly. The splitting of slabs is the most interesting 

 part of their preparation for market. A block about four 

 inches thick and measuring on its face two and one half feet 

 by five feet is placed on end and split into four slabs, each one 

 inch thick. The slabs are put upon revolving platforms and 

 ground to the desired thickness and smoothness of surface. 



Slaty rock, which is not strong and readily crumbles under 

 comparatively light pressure, is called shale. Slate was once 

 shale ; shale was once mud ; and the mud beds were deposited 

 by water as sediment derived from the disintegration of other 

 rocks. 



195. Granite. The story of granite is quite different from 

 that of slate and sandstone. Examination of a piece of granite 

 shows that it is made up of several different kinds of minerals 

 which are usually in crystalline form. The glassy granular 

 particles which do not have the appearance of crystals in 

 granite are quartz. The white, gray, or salmon-colored crys- 

 tals, usually the most abundant of the constituents, are felspar. 

 And the black glistening crystals are either mica or hornblende. 

 Granite in which hornblende occurs instead of mica is called 

 hornblende-granite. When felspar is in the form of large 

 crystals embedded in a finer crystalline mass, the rock is de- 

 scribed as porphyritic granite. Granite is variously colored 

 and takes on a high polish. 



Because of its structure and hardness granite is difficult to 

 work and is correspondingly expensive. Its crushing strength 

 is so great that it is often used in foundations for the heaviest 

 structures. 



Granite is very widely distributed. It occurs extensively 

 not only at or near the surface, but perhaps as the great mass 

 of rock structure of the deeper part of the crust. In mountain 

 systems as the Sierra Nevada or the Himalayas, granite forms 

 the core and appears in such magnificent " dome " structures as 

 those in the Yosemite Valley. The giant monoliths that break 



