Till ORIGIN "I MINERALS j:l 



micas, pyroxene, amphibole, epidote, spinels, a,nd andalusite 

 are minerals which form under high pressures; the hitter are dense 

 minerals and differ entirely from mineral? formed at or near the 

 surface, \\here hydrated minerals are the rule, a class quite un.-table 

 under high pressures and heat. Under such conditions the latter are 

 quickly transformed to the more stable compact molecule. If the 

 minerals which have been formed at depths are brought to the sur- 

 face, they become the less stable phase, when they are subject to 

 change and decomposition, either through the solution of some of 

 their constituents, or oxidation, or by replacement of some ele- 

 ments by others. 



The action of ground waters. Water falling as rain passes 

 through and over the soil, following the course of least resistance, 

 constantly taking up and carrying along soluble compounds. As 

 CO2 is absorbed from the soil and the air, its solvent power on other 

 carbonates, as calcium carbonate, is thereby increased ; such car- 

 bonates in solution are considered as the bicarbonates and are 

 more soluble than the normal salts. 



There is an area near the surface, usually termed the area of oxi- 

 dation or weathering, in which the pores and cavities of the soil 

 and rocks are not completely filled by the percolating waters. In 

 t his area oxygen is an active agent, and many minerals are oxidized 

 as they are carried in solution, or oxidation is the cause of precipita- 

 tion. In this area sulphides, as pyrite, are oxidized to sulphates, 

 forming ferric and ferrous sulphates, both of which are soluble and 

 carried in solution by the descending ground waters, to act as power- 

 ful reagents in the transformation and solution of other minerals. 

 In the area of oxidation, such processes as carbonation and hydra- 

 tion, in addition to oxidation and solution, are active, resulting 

 in the formation of carbonates and many hydrated minerals and 

 hydroxides, as limonite and the oxides of manganese, and alumin- 

 ium, as well as sulphates, arsenates, and phosphates. In this region 

 carbon dioxide can replace silica, the silica being carried in solu- 

 tion to be again deposited either as the amorphous form, opal, or as 

 quartz, according to conditions. This reaction is the reverse of 

 that which takes place at greater depths and under the influence of 

 heat and pressure, where silica replaces CO2. Upon the whole 

 minerals in the zone of oxidation or weathering are decomposed and 

 disintegrated, forming products which with the addition of water 

 and oxygen have increased in volume, and in the production of which 

 hydration and oxidation are most important processes. 



