THE WORK OF GROUND WATER 



61 



Fig. 36. — Quartz veins (white) in schist, 

 near Williamstown, Massachusetts. 



ing another in its place. Shells, 

 bones, and trees are petrified by 

 the replacement, molecule by 

 molecule, of the original substance 

 by mineral matter. (3) Deposi- 

 tion occurs when minerals are 

 taken from the rock in one place 

 and later deposited elsewhere. 

 In this way many metallic and 

 other veins (Fig. 36) are formed 

 (p. 371), and loose sands and 

 clays are cemented into hard 

 rocks. Besides this more impor- 

 tant work, concretions (p. 75) 

 and geodes (p. 78) are formed, 

 and in regions of thick limestone 

 cave deposits are built up (p. 70). 



Belts of Weathering and Cem- 

 entation. — The belt of weathering 

 extends from the surface of the 

 ground to the level of underground water and is of variable thick- 

 ness. In this belt the greatest chemical decomposition of rocks 

 occurs. This work consists mainly in hydration, oxidation, absorp- 

 tion of carbon dioxide, and solution, and it is here that minerals 

 with complex molecules are broken down into simpler compounds. 

 This belt is, therefore, that portion of the earth's crust which is being 

 prepared for its ultimate disintegration into soil. Great porosity, 

 low temperature, and low pressure characterize this zone. 



The belt of cementation is beneath the level of underground water. 

 In this belt, as the name implies, deposition rather than solution 

 plays the leading part. The consolidation of sands and clays into 

 hard rock is brought about here, both by the deposition of minerals 

 obtained by solution from the belt of weathering and also by the 

 pressure of the overlying rocks. The rocks of this deeper zone are 

 more or less porous and fractured, and the temperature is compara- 

 tively low. 



As the surface of the land is lowered by erosion, the belt of 

 weathering invades the belt of cementation, and the minerals which 

 were deposited in the pores and cracks of the latter may again be 

 dissolved out. 



