264 ROCKS AND SOIL 



It is evident that we must think of igneous rock as lying under all 

 other rocks. Time and again some of it has been liquefied, and has 

 penetrated into the cracks of stratified rocks, or has overflowed them. 

 As a result the stratified rocks near by were heated, and changed in 

 structure : they became metamorphic. 



289. Weathering of Rocks. The earth's solid crust at 

 any given time is what it is because of two great agencies : 

 (1) the upbuilding agencies, and (2) the down- tearing 

 agencies. It is as though you were to set one man at 

 work building up your house, and another man at tearing 

 it down. The size and shape of your house would depend 

 upon the rates of speed at which the two men worked. 

 So it is with the earth. Agencies are at work building up 

 the land: igneous rock is pushed up from beneath, and 

 stratified rock is formed by deposit. But no sooner is 

 rock brought to the surface than it is attacked by the 

 air, by the surface water, and by plants and animals. In 

 this way bed rock, whether stratified or igneous, is 

 changed to mantle rock, and mantle rock to soil; then soil 

 is carried away by wind, rain, and stream, and deposited 

 as sediment; the process of rock making begins over again. 

 The action of air and of the "weather" upon the crust of 

 the earth is called weathering. 



Illustrations of weathering can be seen wherever rock (or any mate- 

 rial) is exposed to the air. At the base of every cliff there is a heap of 

 detritus, or talus, that has been broken off from the rock above. Around 

 every boulder in a field, if the ground is undisturbed, you can find chips 

 and dust that were once parts of the boulder. Polished marble 

 monuments become worn and rough, and limestone walls soon have 

 a surface that you can rub off. Artificial weathering is shown in the 

 wearing off of stone and cement sidewalks, floors, and steps, from 

 constant use. 



