78 PHYSICAL GEOLOGY 



depth must be tightly closed. Into this deep zone, then, 

 water cannot readily penetrate, because there are no cracks 

 and pores to afford it passage. If crustal movements take 

 place, these deeply buried rocks cannot break, but the over- 

 whelming pressure forces them to yield or flow like a plastic 

 mass of putty. This deep zone has therefore been called the 

 zone of flowage. 



At and near the surface almost any rock is strong enough 

 to maintain open fissures, cavities, and pores. Crustal move- 

 ments there tend to crack the rocks, the pressure being in- 

 sufficient to mold them. The upper zone is therefore called 

 the zone of fracture. 



Between these two zones is a transition zone in which the 

 weaker rocks, such as shale, yield to the pressure; while 

 strong rocks, like quartzite and granite, remain rigid and may 

 support open fissures. 



How ROCKS ARE ALTERED 



Like most other substances, rocks may be radically changed 

 by subjecting them to great pressure, heat, and the solvent 

 action of water. Some of these alterations result in the decay 

 of the rocks, some in further hardening, and some in a reor- 

 ganization of the minerals of which the rock is composed. 

 These changes are in turn the cause of differences in the color, 

 strength, structure, and texture of the rock. A red sandstone 

 may be changed into a hard, black quartzite ; a massive basalt 

 may become a slaty green schist ; and a dense limestone may 

 be altered to a coarse-grained marble. 



Alterations in the zone of fracture. The rain water 

 which sinks into the soil accumulates in the pores and cracks 

 in the rocks beneath. Except near the surface it completely 

 saturates the rocks, and the body of water thus formed is the 

 source of our wells and springs. At first the water goes 

 chiefly downward ; but soon it begins to travel devious paths 

 through cracks and porous layers, many of which lead it in 



