18 



GEOLOGY 



agonal prism. Similar radiating cracks from other centers com- 

 plete the columns (B, Fig. 8). A five-sided column would arise 

 from the failure of the cracks to develop about some one of the 



Fig. 8. Diagrams to illustrate the formation of columns of basalt: A, the 

 first stage in the development of a hexagonal column. B, the completion 

 of a hexagonal column. C, a pentagonal column. 



points (C, Fig. 8). All igneous rocks are likely to be affected by 

 cracks or joints, which run through them in various directions, but 

 joints are not peculiar to igneous rocks. 



Fig. 9. Granitic rock, about half natural size. The white patches ivpn>- 

 resent crystals of one or two kinds of mineral, and the dark purls repre- 

 sent crystals of others. 



