THE GLACIAL PERIOD 



69 



substance of definite or nearly definite chemical composition, and 

 it crystallizes into regular and constant form. Thus quartz, one 

 of the most common of all minerals, is an oxide of silicon, SiO 2 , 

 with some water added if in crystalline form. The crystals are 

 six-sided prisms with six-sided pyramids at each end, if perfect 

 (Fig. 46). This mineral may occur in great masses or it may 



FIG. 47. Piece of orthoclase feldspar showing cleavage 



be merely one constituent of complex rocks. It is, for instance, 

 an essential element in granite. 



Not many minerals occur as conspicuous ingredients in the 

 rocks of the Chicago region. It would be well to study these 

 few in the collections to be found in the local museums like that 

 of the Chicago Academy of Science or the Field Museum so as 

 to be able to recognize them. Small collections may be had 

 cheaply from any of the well-known dealers like Ward's Natural 

 Science Establishment, Rochester, New York. The following 



