82 



PHYSICAL GEOLOGY 



aspect (Figs. 62 and 63). 

 Where the cleavage planes of 

 the minerals are parallel to 

 the crystals, the whole rock 

 may split readily along them, 

 and the result is schist, if the 

 crystals are large enough to 

 see, or slate, if the rock is 

 dense. 



The result of these pro- 

 cesses is a more or less com- 

 FIG. 62. -Gabbro as it appears under P lete change in the character 



the microscope. The white bodies of the rock. Although the 

 are feldspar, the shaded bodies au- , , j j 



gite, and the black spots magnetite, mashing and grinding down 



The irregular forms of the crystals of the particles tend to pro- 



duce a loose, fine-grained 

 rock, the crystallizing of the 



material, on the other hand, produces a firm, coarser-grained 



rock, which also is usually banded or cleavable. 

 Rocks which are invaded 



by hot lavas from below 



are subjected locally to 



conditions not unlike those 



of the zone of flowage. 



Under the influence of the 



high temperature and of 



the hot solutions and gases 



which emanate from the 



lava, many rocks recrys- 



tallize and undergo other 



radical changes. A lime- 

 stone may become a hard 



Crystalline rock charged FIG. 63. -Schist as it appears under the 



* microscope. Ihe black, white, and 



with Crystals of garnet, shaded bodies are mineral particles of 



hornblende, and other new ", Afferent kinds. 



What would be the direction of cleav- 



minerals. Shale may be age in this rock ? 



