470 ROLLIN D. SALISBURY 



of the solvent work of water, for most mineral matters carried to the 

 sea in solution by rivers are extracted from the water about as rapidly 

 as they are supplied. It is probable, indeed, that the amount of 

 mineral matter which has been extracted from the sea water far 

 exceeds all that remains in solution. This conclusion may be reached 

 either (i) by calculating the volume of rock material which has 

 been extracted from the sea water, or (2) by comparing the pro- 

 portions of the various sorts of mineral matter in sea and in river 

 water. 



1. The rock matter extracted from the sea includes most of the 

 limestone, the gypsum, and the salt, and much of the cementing 

 material of all other sorts of sedimentary rock. Data concerning 

 the thickness of such materials beneath the sea are not available, but 

 some calculations concerning their average thickness in land areas, 

 most of which have been beneath the sea at times, have been made. 

 Dana estimated the average thickness of limestone (presumably for 

 the continents) at about 1,000 feet. 1 This figure appears to take no 

 account of the calcium carbonate which forms an important constitu- 

 ent of many shales and some sandstones, and it is not clear whether 

 it was meant to include dolomites. 



Reade estimates the thickness of the limestones for the globe 

 (not for the continents merely) at about 528 feet; 2 but this figure, 

 like the preceding, appears to take no account of the calcium car- 

 bonate in the sandstones and shales. 



Van Hise, attempting to underestimate rather than overestimate, 

 assigns a thickness of 328 feet to the limestones of the continents in 

 the zone of katamorphism, and estimates that about an equal amount 

 of calcium carbonate exists in the shales and sandstones. 3 This gives 

 an aggregate of 656 feet of calcium carbonate for the continents 

 above the zone of anamorphism. 



If the estimates of Dana and Reade be increased to make allow- 

 ance for the calcium carbonate in the shales and sandstones, and if the 

 estimate of Van Hise be increased to include the limestones below 



1 Dana, Manual of Geology, 4th ed., p. 485. 



2 T. Mellard Reade, Chemical Denudation in Relation to Geological Time (London, 

 1879), P- 53- 



3 Monograph 47, U. S. Geological Survey, pp. 940, 941. 



