AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 273 

 VI. — THE POTASH OF THE RIVERS. 



The matters referred to in the last section lead to the consideration 

 of an objection — which may be iirg-ed against the present thesis — on 

 the inconsistent relations of the alkalies as estimated in the rivers and 

 in the ocean, an objection which has led to the inference that the river 

 discharge of to-day must be different in its chemical nature from what 

 it was in the past. We hope to show that this conclusion is arrived at 

 without consideration of the whole facts, and that in truth the record 

 of the rocks is best to be explained by assuming that this river supply 

 was maintained in the past. 



The potassium in the ocean, on the most reliable estimate of the mass 

 of the latter, amounts to 565 X 10^" tons; converted to K^O, this becomes 

 683 X 10'" tons. The mass of Na^O in the sea is 21 X lU'^ tons. This is 

 a ratio of 1 to 31, nearly. 



On the other hand, the annual river discharge of K.^O is 7.3X10^, 

 and the soda discharge about 21 X 10^, or a ratio of about 1 to 2.8. We 

 must, then, suppose that the rivers are now supplying more potash rel- 

 atively to soda than formerly, or that some process of abstraction of 

 the potash from the ocean is in continual progress. Before consider- 

 ing the last alternative we will examine the first, to determine if such 

 an explanation will clear away the difficulty. 



A deficiency merely due to the river supply having increased in 

 recent periods should not interfere with effecting such a restoration of 

 the potash percentage of the sedimentary detrital rocks as we effected 

 in the case of the soda. Let us see if we can effect such a restoration. 

 We take the most probable estimate of the sedimentary rocks and that 

 most favorable to the restoration. 



This afforded 64X10'' tons of detrital rock and 91x10'" tons as the 

 mass of the parent rock. The potash percentage obtained from Rosen- 

 busch's tables of sedimentary rocks amounted to 2.49. Hence we find 

 1.594X 10'' tons of K.O in the detrital rocks, and adding 663 X 10" tons 

 contained in the sea we get 1.66x10". This, on the total mass of 

 91 X 10", is 1.82 per cent. But from Clarke's estimate of the chemical 

 composition of the original crust this should be 2.83, or about 1 per 

 cent of the potash is missing. On this same estimate of the sedimen- 

 tary rocks the soda percentage comes out correct within O.-t per cent, 

 and for that we have the great deposits of rock salt as a partial set off'. 



It is apparent, therefore, that the actual amount in the ocean disa- 

 grees with the indications of the rocks, and in the same direction — that 

 of deficiency — as it does with the indications of the rivers. 



The missing potash, if we assume the deficiency to be 1 per cent, 



would amount to 91x10'* tons. Assume that this is contained in the 



oceanic precipitates now forming the ocean floor, and add it to the 



KjO now in solution in the ocean, or, more accurately, what is calcu- 



SM 99 18 



