PHYSICAL CHANGES OF THE OUTER SHELL 77 



As noted in the preceding Chapter, lavas which have suc- 

 ceeded in reaching the known outer part of the lithosphere 

 produce various structures and effects. They may solidify 

 beneath the surface in bodies of vast size; they may bulge 

 the overlying rocks in blister-like form ; they may fill cracks 

 and bedding planes; and they may even reach the surface, 

 there to be poured out as lava flows or be blown into dust 

 and cinders. Doubtless that part which solidifies in the form 

 of batholiths, stocks, and laccoliths far exceeds the portion 

 which has been built into surface plateaus and the familiar 

 volcanic cones. All of these are effects of one great process, 

 vulcanism. 



The building of characteristic structures and the formation 

 of igneous rocks is not the only conspicuous effect of vulcan- 

 ism. The lavas may bake the rocks which they penetrate. 

 The hot gases and solutions, which most bodies of lava emit 

 as they cool and crystallize, spread out through cracks and 

 pores in the surrounding rocks and deposit quantities of 

 minerals which they originally held in solution. By these 

 means the country rock may be considerably altered. 



ZONES OF FRACTURE AND FLOW AGE 



As compared with other things familiar to us, rock is one of 

 the hardest and strongest materials. Nevertheless, as we 

 have already intimated in describing folds, there is a limit to 

 its strength. 



At the surface the rock lies under the weight of the 

 atmosphere only (about 15 pounds per square inch). At 

 a depth of one mile there is added to this the weight of 

 a column of rock one mile high, or about 6300 pounds per 

 square inch. This is enough to crush the softer limestones 

 and sandstones. At a depth of 6 miles or more the pressure 

 is so much greater that even the strongest rocks cannot resist 

 it. Each grain is compressed into the smallest space it will 

 occupy, and any cavities or pores existing in rocks at that 



