940 A TREATISE ON METAMORPHISM. 



On the supposition that the sediments of the zone of katamorphism 

 are 2 kilometers thick, they compose about 3.668 per cent of the volume 

 of the lithosphere to a depth of 16.1 kilometers (10 miles) below the level 

 of the sea (6,815,000,000 cubic kilometers). With a specific gravity of 2.7 

 the weight of the sediments would be 675,000,000,000,000,000 metric tons. 



In suggesting the above figures the aim has been to underestimate 

 rather than to overestimate the total amount of the sediments. It is thought 

 better to have the calculated amount of elements required for certain 

 changes smaller than the real amounts rather than larger. For instance, 

 it will be explained on the following pages that the change of ferrous to 

 ferric oxide in the sediments has required a vast amount of oxygen — indeed, 

 an amount so great as to be a large percentage of the total oxygen now 

 present in the atmosphere. If an overestimate of the total mass of 

 sediments were made, this would lead to an overestimate of the amount of 

 oxygen required for this change. But the purpose is to make the estimate 

 of the amount of oxygen and of the other elements required for this and 

 for other changes less than the real amounts rather than more. 



In making the first rough approximation of the weight of the sediments, 

 they will all be supposed to have a specific gravity of 2.7. While the 

 specific gravity of sediments varies, and doubtless upon the average is 

 .somewhat less than 2.7 (the average of the igneous and crystalline rocks), 

 the error in placing it at 2.7 is small compared with the errors in the 

 estimates of the amount of sediments and their relative proportion. There- 

 fore, for convenience in the present provisional calculations, a uniform 

 specific gravity for the rocks of the lithosphere is assumed. If the sediments 

 of the zone of katamorphism were supposed to be 0.65 shales and other 

 rocks largely composed of silicates (pelites and psephites), 0.30 sandstones, 

 and 0.05 limestones, this would give 162,500,000 cubic kilometers of shales, 

 weighing 438,750,000,000,000.000 metric tons, 75,000,000 cubic kilometers 

 of sandstones, weighing 202,500,000,000,000,000 metric tons, and 12,500,000 

 cubic kilometers of limestones, weighing 33,750,000,000,000,000 metric 

 tons. In the following pages the term shale will be understood to include 

 all the clastic sedimentary rocks rich in silicates. 



So far as I know there has been no attempt to estimate accurately the 

 relative quantities of shales, sandstones, and limestones. The above parti- 

 tion of the sediments is the roughest sort of a guess. The only computation 



