480 ROLLIN D. SALISBURY 



oxide, 1 the amount of calcium carbonate produced would be about 

 490,900,000 tons per year. This would make approximately ^ of 

 a cubic mile. To furnish the amount of calcium oxide necessary 

 for this amount of calcium carbonate, the decomposition of nearly 

 one-half (0.44) a cubic mile of average rock would be required. In 

 370,000,000 years (p. 474) some 163,000,000 cubic miles of average 

 rock would have been decomposed at this rate. 



4. We are not obliged to rely on calculations based on somewhat 

 unobtrusive changes, for our knowledge of the wasting away of the 

 continents. Some conception of its importance may be gained in 

 another way. The amount of sediment which streams carry to the 

 sea each year has been calculated, with some approximation to 

 accuracy. The amount of matter which they take to the sea from the 

 land, including that carried in solution, as well as that carried 

 mechanically, has been estimated as 3. 7 cubic miles every year. 2 If 

 this figure be modified to make allowance for the reduced volume in 

 solution (p. 473), it becomes a little less than 3 cubic miles (2.93). 

 Besides this loss to the lands through the erosion of the rivers, the 

 winds blow great quantities of dust and sand into the sea every year, 

 while the waves beat effectively on the coasts, cutting off, in the 

 aggregate, enormous quantities of land material, and extending the 

 dominion of the sea. 



If rivers were to continue to wash away the continents at the pres- 

 ent rate, they would remove to the sea an amount of material equal 

 to all that is now above sea-level in less than 8,000,000 years, and the 

 work of the winds and waves would shorten this period considerably. 



If rivers have been wasting the land at the present rate for 370,- 

 000,000 years, they would have destroyed and carried away from the 

 land about 58 times the amount of rock material now above sea- 

 level. These figures take no account of the material blown from 

 the land to the sea, nor of that worn from the shores by waves. If 

 the work of these agents were taken into consideration, the figures 

 given above would be notably increased. On the other hand, the 



1 F. W. Clark, has shown that the ratio of calcium oxide and magnesium oxide 

 in 345 limestones analyzed is about 5^ to 1. Bulletin 228, U. S. Geological Survey, 

 pp. 20, 21. , 



* Murray, op. cit., Vol. IV, p. 41. 



