MATERIALS OF THE EARTH 



53 



of igneous rock. In all such cases pressure probably attends the 

 heat, and is a factor in the development of the schists. 



\\lien the change induced by the heat is less considerable, the 

 shale is baked, with incipient re-crystallization, and often takes 

 the form of argillite, a compact, massive sort of shale. 



Beds of hydrous iron oxide (limonite) or of iron carbonate (sider- 

 ite) may be converted by heat into hematite or magnetite. Beds 



Fig. 29. Figure showing the elongation of pebbles resulting from pressure. 

 Carboniferous formation, Bancroft Place, Newport, R. I. (Walcott, 

 U. S. Geol. Surv.) 



of peat, lignite, and bituminous coal are converted into anthracite 

 by the driving off of the volatile hydrocarbons. If the process 

 goes to the extreme, graphite is the result. 



Metamorphism by pressure. Slaty structure. When rocks 

 made up of clastic particles are compressed in one direction, and 

 are relatively free to expand in others, the particles that are already 

 elongated tend to turn so that their longer axes are at right angles 

 to the direction of pressure, and all particles, whether elongate 



