24 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



their filling results in the formation of fissure veins. In case solution 

 furnishes the requisite receptacle, which in general, as it seems, becomes 

 filled as rapidly as formed, concentration by replacement results. 



The great group of concentrates just referred to results from chemical 

 precipitation under the direct and immediate control of physical con- 

 ditions, chief among which are variations in temperature and pressure. 

 So complex are the conditions leading to the filling of cavities in the 

 zone of cementation and so little are they understood that it does not 

 seem practicable to classify the results in terms of the agencies in 

 operation. In order to make convenient subdivisions among the results, 

 produced, resort must seemingly be had to the character of the recep- 

 tacles in which concentrates have been placed, as has been done in classi- 

 fying mineral veins, geodes, agates, amygdules, etcetera, or else the 

 mineral composition of the concentrates themselves employed for a like- 

 purpose and various ores and mineral deposits recognized. As is well 

 known, each of these methods has been employed, but without formula- 

 tion of a definite system of classification. 



In the zone of cementation, as has been mentioned, the temperature 

 is normally below the critical temperature for water. A higher tempera- 

 ture, especially if the fusion of rock results, leads in general to the 

 diffusion and not to the concentration of mineral matter. Under certain, 

 conditions, however, when heterogeneous material is in a fused condition,, 

 or when part of a magma crystallizes, leaving other portions still liquid,, 

 gravitation may lead to the separation of lighter from heavier magmas, 

 or to the settling of solid products from a cooling magma, or from one in 

 which decrease of pressure has favored partial crystallization. These two 

 processes, although distinct, depend on selective gravitation and may 

 progress at the same time. The nature of the magmas composing the- 

 earth's highly heated interior is known in part from the study of the 

 extrusions of plastic or liquid rock discharged by volcanoes, but in 

 general it is only the cooled and crystallized material which was present 

 in the earth's centrosphere and has been forced outward that is available- 

 for examination. 



The study of igneous rocks has shown that concentration during the- 

 process of cooling has resulted in the formation of minerals, but in 

 general these are not gathered into groups or masses, but disseminated 

 through the rock. The layers of the disseminated concentrates are- 

 crystals of various minerals, and the ones of commercial value are prin- 

 cipally feldspar and certain gems. In certain instances, however, as 

 in the case of pegmatite veins, original concentration from a magma 

 seems to have occurred; but even in these instances later concentration 



