232 CLARKE— SOME GEOCHEMICAL STATISTICS. [April 20, 



already been shown that the limestones, which are mostly of marine 

 origin, have a volume of 3,916,400 cubic miles. With a specific 

 gravity of 2.7 their mass becomes 42,092 X 10^" metric tons. From 

 the figures given on p. 230 ante, the calcareous and magnesian sedi- 

 ments are now forming at a rate bearing a certain ratio to that of 

 the other deposits, the limonitic and siliceous residues. This ratio, 

 which is roundly 1,650:452, if constant throughout geologic time, 

 would give for the latter class of sediments, proportional to the 

 limestones, a mass of 11,664 X 10^- tons; the sum of both classes of 

 precipitates being 53,756 X 10^^ tons. The corresponding average 

 thickness over the sedimentary oceanic area would then be 287 feet, 

 or less than one third of the figure previously given. The actual 

 thickness, however, must be much less ; for a large part of the once 

 marine sediments are now elevated into land. According to the best 

 estimates, ^^ the land area of the globe is now covered by 23 per cent, 

 of archsean and eruptive rocks, and yy per cent, of sedimentaries. 

 Adding this sedimentary area to that of the ocean, the total becomes 

 132,180,000 square miles, and the average thickness of the chemical 

 sediments reduces to 191 feet. At the crude value assigned to 

 geologic time this represents a rate of deposition of only 0.000027 

 inch annually. If the age of the earth is less than 83,472,000 years, 

 the mean annual rate of deposition will be proportionately increased, 

 but not to anything like the present magnitude. 



Whether the ratio assumed between the calcareous and siliceous 

 sediments is justifiable or not, is a question admitting of argument. 

 It seems, however, probable, that in the earliest geologic ages, when 

 the land area was occupied principally by igneous rocks, the salinity 

 of the rivers was relatively low, but the proportion of silica to lime 

 in the waters was higher. This suspicion is justified by a study of 

 the river waters of today, especially those issuing from granitoid 

 areas. In such waters silica is often in excess of lime, while in 

 waters from sedimentary areas the reverse is commonly true. The 

 ratio here assumed represents a balancing between waters of both 

 classes, and is therefore as legitimate as any other which might be 

 chosen.. Here it must be borne in mind that we are dealing with 

 probabilities only, nothing more. 



"Von Tillo as modified by Becker. See Becker's memoir already cited. 



