TRANSPORTATION OF THE METALS. 1079 



volume to the point where they begin to escape laterally. (See fig. 26.) 

 Hence in a trunk channel of a definite size the circulation is slow below and 

 increases in speed above. Near the bases of the channels from which the 

 Mammoth Hot Springs and geysers of the Yellowstone Park issue the 

 amount of water contributed may be small and the movement of the water 

 may be exceedingly slow. Even if true, as held by some, that rapid 

 movement of water is unfavorable to deposition of ores, it is wholly possi- 

 ble that at moderate depth, especially in the deeper parts of a channel from 

 which the flow at the surface is rapid, the conditions are those of slow move- 

 ment and rapid precipitation of ore deposits. 



As the water passes upward the variety of solutions as well as the 

 amount increases, for each stream differs in its salts from every other, since 

 no two streams can possibly have had exactly similar histories. Moreover; 

 the character of the wall rock may vary from place to place. The pressure 

 and the temperature also lessen. These conditions are favorable to precip- 

 itation. Therefore many ores in their first concentration are precipitated by 

 ascending waters. 



It is now clear that a satisfactory account of the genesis of ores includes 

 ascending waters. Many ores in their first concentration are actually pre- 

 cipitated from the ascending waters, and therefore emphasis has been placed 

 upon this part of the work of circulating waters. 



The broader statement of the genesis of a great class of ore deposits is 

 that the water after penetrating the earth is widely scattered in contact with 

 rocks in innumerable minor openings. These waters travel downward with 

 steadily increasing pressure and temperature. They take up the constitu- 

 ents of the ore deposits. The downward movement of the waters has 

 superimposed upon it a lateral component, as a result of which the waters 

 are carried to the larger openings,. During this process also the waters 

 continue to take material into solution. In the larger openings where the 

 waters are congregated they are upon the average ascending with decreasing 

 temperature and pressure, and the ores are precipitated. 



While it is believed that in the great majority of instances the journey 

 of the underground water involves first a descending and later an ascending- 

 movement, it is recognized that this is not invariably the case. Upon the 

 descending movement may be superimposed a lateral movement which 

 brings the water _to the surface again at a lower level, there being at no 



