PHYSICAL-CHEMICAL CONCENTRATION 23 



exceptions are in association with the outward migration of heated 

 waters or of magmas from deep within the earth. It is in the inner 

 earth from which the igneous rocks now at the surface have migrated 

 that the earth's great physical-chemical laboratory is located and those 

 geological processes requiring high temperatures and great pressure are 

 carried on. In the lithosphere below the zone of weathering two im- 

 portant divisions in reference to the processes of concentration may be 

 recognized — an outer zone, in which water may exist in cavities and also 

 because the temperature is not sufficiently high to reach the critical tem- 

 perature above which the elements of water are disassociated, and an 

 iimer zone, or the earth's centrosphere, in which these conditions are 

 reversed. 



The zone beneath the zone of weathering is a zone of saturation, 

 whose waters linger or in general migrate but slowly, and where also 

 temperatures are high and pressure enormous and progressively increasing 

 with depth. Under these conditions the chemical reactions are favored 

 and the results, so far as concentration of mineral matter is concerned, 

 are free from the complications that arise when evaporation, sublimation, 

 and influences of life are active. This zone is thus especially favorable 

 for the production of mineral concentrates through the process of chemi- 

 cal reaction, as has already been emphasized by Van Hise, who has 

 named it the zone of cementation. In the zone of cementation cool de- 

 scending waters meet and commingle with heated ascending waters, and 

 lateral migration of solutions involving changes of temperatures and 

 pressure also occur. The fact that in this zone, which is in a condition 

 of saturation, the water present is under pressure and subject to the 

 laws of hydrodynamics, and in consequence will move from regions of 

 greater' to regions of less pressure, irrespective of direction, is sometimes 

 lost sight of. Possibly also in the case of highly heated and saturated 

 rocks the principal governing the diffusion of solutions through mem- 

 branes or endosmosis may play a part in the migration of matter. 



Under the general conditions named — that is, high temperature, satu- 

 ration, slow movement of water, and hence abundant time for chemical 

 processes to act and great pressure — the one thing that controls con- 

 centration is the presence of suitable receptacles in which the materials 

 precipitated may be accumulated and preserved. Such receptacles, as 

 is attested by mineral veins, are furnished by cavities in the rocks. 

 When the cavities are small and in general evenly distributed, we term 

 the process of concentration of mineral matter cementation. When the 

 cavities are larger and cave-like in character, geodes and agates result. 

 When the cavities are produced by fracture and open fissures result, 



