PHYSICAL CHANGES OF THE OUTER SHELL 79 



horizontal or diagonal directions. Afterward a part of it 

 may issue at lower points in springs. To this pervasive cir- 

 culation of water are due many of the most important changes 

 which rocks undergo. 



In its early downward course, percolating water dissolves 

 out of the rocks the more soluble materials. This renders the 

 rocks more porous and decayed, until they crumble, leaving 

 the less soluble sand and clay to form the soil. These in turn 

 are likely to be carried off by winds, streams, and other 

 agencies and deposited as beds of sand and mud, which may 

 eventually become sandstone and shale. 



In the course of its descending journey the water thus be- 

 comes saturated with various mineral materials. When in 

 this state a slight change in 

 the temperature, or other 

 surrounding conditions, may 

 be sufficient to cause the 

 deposition of some of the 

 dissolved substance. Thus 

 quartz may crystallize out 

 of a solution which is slowly 

 seeping through a porous 

 bed of sandstone and may, 

 by filling up the pores, pro- 

 duce a firmly cemented 

 quartzite (Fig. 60). Only a 

 part of the mineral matter 

 taken into solution during 

 the decay of the rocks is 

 used up in this way; some of it is carried out through springs, 

 joins rivers, and is finally poured into the sea. 



Minerals often crystallize upon the walls of a fissure through 

 which a water solution is rising, and by this process the crack may be 

 completely filled. The result is a mineral vein. The commonest 

 vein minerals are quartz and caleite ; but occasionally rare and 

 valuable minerals, such as gold, silver, compounds of lead, zinc, and 



FIG. 60. Quartzite as it appears 

 under the microscope. The individ- 

 ual sand grains may still be identi- 

 fied by their rounded outlines, but 

 the interspaces are completely filled 

 with quartz. 



