THE EARTH'S ROCK FOUNDATIONS 73 



Sandstone is composed of sand grains more or less thoroughly 

 cemented together, and may be quite hard or very soft and 

 friable. It often contains impurities, notably the oxides of 

 iron that impart various colors to it, chiefly yellow or red. It is 

 the result of the solidification of old beds of sand. Conglomerate 

 is merely a very coarse sandstone in which the component bits 

 are rounded, water- worn pebbles instead of sand grains. Breccia 

 is similar, but the bits of stone of which it is formed are still 

 angular. 



Beds of clay when transformed to rock by pressure and heat 

 form shales. They are fine-grained rocks, usually soft and split 

 easily into layers. They give an earthy odor when breathed 

 upon. They also vary greatly in color, depending on the nature 

 of the contained impurities. 



The soft coals are also sedimentary rocks. Along the margins 

 of the ancient seas there occurred sometimes extensive swamps, 

 especially at the mouths of rivers just as they exist today in 

 deltas. In these vegetation was very rank. As the trees, ferns, 

 rushes, and other forms matured and fell, they sank into the 

 shallow water which covered them and prevented their immediate 

 decay. Year after year, century after century, added to the 

 accumulation until the lower layers were compressed into peat. 

 So peat beds are forming nowadays in such locations. These 

 peat beds continued to form to great depths, the crust of the 

 earth sinking with the weight of the great accumulation. The 

 lower layers were still more powerfully compressed by the great 

 weight above them, and were heated from the earth's hot interior. 

 So lignite or brown coal was formed, and this in turn changed to 

 bituminous or soft coal as the gases and more volatile oils were 

 driven off. Often quantities of mud were brought down by the 

 rivers and deposited in such swamps. When compression 

 occurred these transformed to shales. Since the clay contained 

 much vegetable material, the shale formed from it is dark, 

 carbonaceous shale. It often is impregnated with the oils and 

 gases that distil off from the forming coal. 



