982 A TREATISE ON METAMORPHISM. 



the sandstones and limestones is less than the average for the original rocks. 

 The increase in the amount of combined water in hydrating the materials 

 of the shales, 0.65 of the sediments, 438,750,000,000,000,000 metric tons, 

 from 1.92 to 5.02 per cent, is 13,601,250,000,000,000 metric tons. The 

 decrease in the combined water in the sandstones, 0.30 of the sediments, 

 202,500,000,000,000,000 cubic kilometers, from 1.92 to 1.69 per cent, is 

 465,750,000,000.000 metric tons. The decrease of the water in the lime- 

 stones, 0.05 of the sediments, 33,750,000,000,000,000 metric tons, from 1.92 

 to 0.98 per cent, is 317,250,000,000,000 metric tons. The total decrease 

 in the sandstones and limestones is 783,000,000,000,000 metric tons, which 

 sum, subtracted from the increase in the amount of water in the shales, 

 leaves a net increase in the water of hydration of the zone of katarnorphism 

 of 12,818,250,000,000,000 metric tons, and an equivalent loss of water to 

 the hydrosphere. 



This estimated amount is probably too small, for the following reasons: 

 Throughout the immense thickness of the zone of katamorphism, 

 hydration is the rule. A very large portion of this zone is composed of 

 igneous rocks, and the minerals of these rocks have been hydrated to a 

 varying extent. In the plutonic rocks which have not been fractured in a 

 complex way the process has usually not gone far, but in the volcanic 

 rocks, and especially the porous ones, such as basalts, hydration has taken 

 place upon a vast scale. For instance, in various parts of the world, such 

 as in the Lake Superior region and in the Deccan, hydration has gone so 

 far in the basic amygdaloidal lavas as in many cases to have completely 

 destroyed the original minerals. One of the results of such alterations is 

 the filling of the pores of the vesicular rocks with various minerals of which 

 hydrous silicates, such as the chlorites, zeolites, etc., are especially abundant. 

 The process of hydration of the zone of katamorphism has continued during 

 geological time. So far as I can see there is no way to estimate the amount 

 of water thus absorbed, but I suspect that the amount abstracted from the 

 hydrosphere by the process is probably greater than that used in the hydra- 

 tion of the sedimentary rocks. 



In consequence of the hydration of the original rocks of the zone of 

 katamorphism, it is certain that the amount of combined water in the 

 igneous and crystalline rocks analyzed in order to determine the compo- 

 sition of the lithosphere is greater than in the recent igneous rocks. The 



