10.80 



A TREATISE ON METAMORPHISM. 



time an important ascending movement. This is especially likely to occur 

 in districts of great topographic relief, where the descending waters strike 

 an inclined impervious plane dipping toward a valley. (See fig. 27.) In 

 the case illustrated the water falling on the mountains percolates downward 

 through the rocks until it comes into contact with an impervious stratum, 

 which it follows toward the valley until it issues above the stream, having 

 nowhere during its journey an ascending component. 



"While it is held, as explained above, that upon the whole the descending 

 waters are dissolving material and the ascending waters are depositing 



Fig. 27.— Ideal section showing underground circulation in which no water anywhere ascends before issuing at the surface. 



material, it is not meant to imply that materials are not deposited by 

 descending and lateral-moving waters, nor that materials are not dissolved 

 by ascending waters. Indeed, it is certain that solution and precipitation 

 are taking place at all times throughout the entire course of all the branches 

 of the underground circulation. This is a necessary consequence of the 

 laws of physical chemistry. It is meant only to imply that in the first con- 

 centration of one class of ore deposits, solution so far as the ores are 

 concerned is the rule for the descent and deposition for the ascent, although 

 there is no doubt that there are many local exceptions to this. 



