MECHANICAL CONCENTBATION 11 



The importance to industry of the results of the processes of concen- 

 tration just considered is illustrated by the manifold uses of stratified 

 deposits of gravel, sand, and clay, by a large class of building stone, 

 metals obtained from placer mines, and by many other accumulations. 



MECHANICAL CONCENTRATION OF LIQUIDS AND GASES 



In the above outline review of the various processes by which mechani- 

 cal agencies lead to the concentration of various classes of products, only 

 matter in the condition of a solid has been considered. The far-reaching 

 principle involved, however, applies to the gathering together under cer- 

 tain conditions of liquids and gases. The extent and importance of this 

 phase of the subject is illustrated not only by lakes, seas, the ocean, 

 ground water, which require only mention at this time, but by certain 

 more special or more exceptional occurrences, such as the accumulation 

 of petroleum and rock gas in porous terranes beneath impervious and at 

 times arched roofs, and in fissures, caverns, and other openings. The 

 process of concentrating gases heavier than air at the earth's surface is 

 exemplified by the accumulation of carbon dioxide in Death gulch, in the 

 Yellowstone National Park, and many other similar instances elsewhere. 



In the case of liquids and gases concentrated in these several ways, 

 either at the earth's surface or in the outer portion of the lithosphere, it 

 is a prerequisite that receiving reservoirs shoiild be present, but in 

 searching for the dominant principle which leads to the transfer of 

 liquids and gases from one locality to another and their accumulation, 

 particularly in commercial quantities, it is evident that gravitation plays 

 the leading part. 



SELECTIVE POWER OF GRAVITATION 



In the various assorting processes controlled principally by gravity a 

 selective power is exercised with reference to certain physical properties 

 and the ignoring of other similar properties. For the most part, gravity 

 takes note of the ratio of weight and volume, but in part makes a selec- 

 tion in reference to form, and again derives assistance from molecular 

 attraction, and from electrical or other but little known or but im- 

 perfectly understood conditions. 



In a negative way it may be said that mechanical processes of concen- 

 tration take no account of chemical composition, crystalline or other 

 structure, or of color, hardness, or elasticity, except so far as their prop- 

 erties affect size, weight, shape, or attraction or repulsion. 



Mechanical concentration is in brief a physical process leading to the 



