428 GEOLOGY. 



charged with different solutes. On coming together, reaction between 

 the constituents takes place, resulting sometimes in new solutions and 

 sometimes in precipitation. 



If these lower deposits of calcite, quartz, sulphides, etc., are made 

 in the pores of the rock, they change its texture and composition. If 

 they are made in fissures they constitute veins, and if a sufficient per- 

 centage of the vein matter consists of valuable metallic compounds, 

 they constitute ores. ^, ..^, 



As the waters descend they suffer greater and greater pressure and 

 some increase of temperature, and these changes modify their power 

 to hold substances in solution. In general, the waters increase in solv- 

 ent power, but the effect is different for different mineral substances, 

 and hence as a rule the waters are taking up some substances and laying 

 down others as they proceed. After penetrating to greater or less 

 depths, the waters may come again to the surface, either because they 

 are pushed up by the higher head of the w^aters behind, or because they 

 become warmer and thus fighter, and are forced up by the heavier cold 

 waters above, or else they pass up by diffusion through the descending 

 waters. In any case, the deep, warm waters, usually rather highly 

 charged with material dissolved in their previous courses, are apt to 

 deposit some of their burden as they ascend to horizons of lower pres- 

 sures and temperatures. They are particularly liable to make deposits 

 where they commingle with other waters differently charged with 

 solutes. Thus internal changes in the body of the rocks are, and for 

 ages have been, taking place. In the upper part of the depositing zone, 

 calcite is the dominant mineral deposited, while in the lower, quartz is 

 more common; but much depends on local conditions and other influences, 

 and no rigid rule holds good. 



Hydration and dehydration. — ^Water sometimes unites directly with 

 some of the constituents of a rock and produces hydrated minerals, i.e., 

 minerals that have water as an element of their constitution, not simply 

 water absorbed into their pores. A large class of minerals known as 

 zeolites, because they swell up and undergo fife-fike contortions when 

 their basic water is driven off by heat, are examples of hydrous prod- 

 ucts. A more famifiar example is limonite (Fe 263,31120), of which 

 yeUow ocher is a variety, which on heating sufficiently gives off its water 

 and becomes hematite (Fe203) or red ocher. The turning of yellow clay 

 to red brick on burning is a famifiar example of dehydration. The 



