CHEMICAL WORK OF WATER 177 



or lithification of the rock masses. (See the section on diagenesis 

 of rock masses, Chapter XIX.) 



Not only are the pores in the rock masses filled, but fissures are 

 closed up by tlie formation of mineral veins. Large cavities, either 

 originally due to some disturbance, or solution-caves, brought into 

 the belt of cementation by a rise of the ground water level, will be 

 slowly filled by crystallized mineral matter. In the lead and zinc 

 district of Missouri such caverns were slowly filling with crystals of 

 calcite before the last change in ground-water level. These crystals 

 have the form of huge scalenohedrons, some of them half a meter or 

 more in length, and they project from the walls in the manner of 

 crystals from the walls of a geode. The ordinary deposits of cav- 

 erns, stalactites and stalagmites, characteristic of the zone of weath- 

 ering, are wholly absent from these caverns, which were but recently 

 exposed, by the lowering, through pumping, of the ground-water 

 level, from a few meters to a depth of 45 to 60 meters (Van Hise- 



The temperature of the water within the belt of cementation may 

 be greatly raised by the presence of masses of hot igneous rock, 

 the time of cooling for which is much greater than that required 

 at the surface. The juvenile waters given ofif by such lava sheets 

 are likewise in a highly heated state, and so accomplish much solu- 

 tion. 



Hydration. This is the chief reaction in the belt of cementa- 

 tion, and is second in importance only to deposition or cementation. 

 Since v/ater is everywhere present, the minerals are constantly ex- 

 posed to it, and hydration as well as solution must result. It is in 

 this belt that the great group of hydrous silicates and oxides, such 

 as the hydromicas, chlorites, zeolites, serpentine, epidote, limonite, 

 and gibbsite form most abundantly. Kaolin and talc also form here, 

 though they are more characteristic of the belt of weathering. Gyp- 

 sum is altered to anhydrite, in the belt of weathering, whereas hy- 

 dration may change anhydrite to gypsum. The change in the proc- 

 ess of hydration involves an increase in volume of from 30 to 50 

 per cent. 



Oxidation. Water entering the soil commonly carries oxygen 

 in solution, the amount varying with the porosity of the soil, lack of 

 vegetation, atmospheric pressure, etc. Oxidation goes on through- 

 out the belt of weathering, but affects only the upper layers of the 

 permanently saturated belt, for the supply of oxygen is quickly ex- 

 hausted, and its replenishing is a slow process. Normally the depth 

 below ground-water level to which oxidation is restricted is not more 

 than a few meters, but in exceptional cases this may go much far- 



