THE EARTH'S ROCK FOUNDATIONS 63 



of the accumulating layers above and to the internal heat of the 

 earth, are transformed to rock. Just as man takes clay and by 

 pressure and heat transforms it into solid brick, so in nature the 

 loose sands and clays are by similar processes transformed to 

 rock. A bed of sand, for instance, will make sandstone. Such 

 rocks, the constituent materials of which are deposited by water 

 and solidified by heat and pressure, are known as sedimentary 

 rocks. 



- FIG. 29. Basalt 



Such processes of rock formation and rock disintegration by 

 weathering and the re-formation from the debris have been going 

 on for a very long time on the earth. The very old rocks, how- 

 ever, are all igneous apparently. The earth was at one time 

 much hotter than now, volcanic activity was more intense, lava 

 outflows were very extensive, and the early crust was made of 

 the rocks obtained by cooling of this molten material. These 

 very old rocks have, in a large measure, been covered up by later 

 outflows of lava and by sedimentary deposits on top of them. 



