252 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



down— is probabl}^ compensated by their physical character, in many 

 cases conferring- greater durabilit}' upon them. These and other con- 

 siderations lead to the view that there is no sufficient evidence to 

 ascribe greater alkali content to the rivers of the past. 



The origin of the interstratified beds of rock salt, the solvent denu- 

 dation of the sea, and the order of magnitude to be ascribed to an 

 allowance for the latter are briefly considered in the paper, as well 

 as other questions which arise. 



When all corrections are made and the requisite latitude of error 

 taken into account, it would appear that the considcn-ation of solvent 

 denudation points to an age for the earth, dating from the settlement 

 of water upon its surface, of between eighty and ninety millions of 

 years. 



I. — THE ESTIMATE OF GEOLOCrlCAL TIME. 



On the basis that the ocean possesses an average depth of 2,000 fathoms 

 and occupies eight-elevenths of the area of the globe, its total mass is 

 calculated to l)e 1.322 X 10^* tons.' Its ingredients in solution arc: 



Chloride of sodium 77. 758 



Chloride of magnesium 10. 878 



Sulphate of magnesium 4. 737 



Sulphate of lime 3. 600 



Sulphate of potassium 2. 465 



Bromide of magnesium 0. 217 



Carbonate of calcium 0. 345 



100. 000 



and the total salts are approximately 3.5 per cent of the mass of the 

 whole. 



On these data the absolute masses of the ingredients of the ocean are 

 calculable: 



NaCl 35,990X1012 tons. 



MgCla 5,034Xl0inons. 



MgSO^ 2, 192X10'^ tons. 



CaSO^ l,666Xl0iHons. 



K^SOi 1, 141 X W^ tons. 



MgBr lOOXlO'Hons. 



CaCOa 160X10'^ tons. 



46,283Xl0inons. 



Of the sodium chloride 39.32 per cent is sodium. In the sea there 

 is therefore a mass of sodium in solution amounting to 14,151 X 10'' 

 tons. 



' Encyclopedia Britannica— Article, ' ' Sea." The analyses are Dittmar's, from the 

 reports of the Challenger expedition. 



