262 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE "EARTH. 



than hydrochloric acid. Its activity has probably extended all through 

 geological time, and its gradual absorption been marked by the lime- 

 stones of all ages. We must bear in mind, however, what has fre- 

 quently been pointed out, that the volcanic extrusion of this substance 

 throughout geological time has probably been considerable. 



The only warranted correction of large amount appears, then, to be 

 that due to the attack upon the early rocks by the hydrochloric acid. 

 We make a deduction of 12.5 millions of years for the primeval salinity 

 of the ocean brought about ])y this means. It is, however, evident 

 that the solution of this amount of material was not effected instan- 

 taneously, but by an accelerated denudation, extending over some 

 period of time, the duration of which, we can satisfy ourselves, how- 

 ever, is nearly negligible. A rough estimate of the amount of denuda- 

 tion, as a layer of rock removed from the whole terrestrial land sur- 

 face, is easily arrived at. 



We have already found, in fact, that 1,972 X 10^^ tons of sodium must, 

 roughly, have been dissolved out of the primeval rocks. In Clarke's 

 table of the atomic ratios of the constituents of these rocks, sodium 

 appears as 2.68 per cent. The entire mass of rock reduced to detrital 

 sediment and brought into solution to supplv this amount can, of 

 course, be estimated from this. It amounts to 73 X lU^' tons. On 

 Professor Wagner's determination already referred to, the area of the 

 globe is about 1,965x10'' square miles; and, taking the specific gravity 

 of the original rock to be that of diorite (2.95), we find the amount 

 denuded would cover the earth to a depth of 157 feet. 



The most careful estimate of the present mean rate of subaerial 

 denudation of eruptive and crystalline rocks amounts to al)out 1 foot 

 in 3,000 years, removed from the surface, partly in solution, partly by 

 transportation, to the rivers and sea. The primeval ocean was, accord- 

 ing to our view, a dilute solution of HCl. 



Bearing in mind the fact that the solution of hydrochloric acid would 

 become impoverished as time progressed and insoluble residues cover 

 up a fraction of the earth's surface, it would seem to be sufficient to 

 assume that its mean rate of activity was five times that of present sub- 

 aerial agents. This affords 1 foot in 600 years, or to denude 157 feet 

 of rock, 91:, 200 years. In fact, even at the present rate of denudation, 

 the large surface we have assumed as exposed to denudation reduces 

 the period of time required to remove such a rock layer to a negligible 

 duration. The only correction that is admissible would be a tmit in 

 the decimal place of our correction of 12.5X10^ years, reducing the 

 correction to 12.1x10'' years, and leaving geological time, dated from 

 the condensation of water upon the globe, to be 87 X 10' years. 



The foregoing corrections involve not only the assumption that the 

 hydrochloric acid was free, but also that we may assume from the 

 mean analyses of igneous, eruptive, and crystalline rocks a knowledge 



