ON rilE GEOLOGICAL AGfi OF THE EARTH. .3/7 



present in tlie abundance thought by some, its early effects were jDrobably 

 compensated by the more active effects of organic acids of later times. 

 Vegetation, too, has been a source of carbonic acid in the soils during 

 subsequent periods. It may be observed that the fixation of free CO^ by 

 vegetation so abundantly in the later Palaeozoic and the increase in the 

 deposits of limestone rather point to a gradual fixation of this substance 

 than to any special activity as a solvent in earlier times. On these 

 accounts we make no correction for the possible presence of this substance 

 in primeval times. 



Rock solution in heated waters persisted for periods relatively so short 

 as to cause negligible error only. According to Lord Kelvin's calculation 

 as to the rate of cooling of the solidifying crust, a period of a century 

 ■would have been adequate to cool the crust from its melting-point down 

 to about 8° Centigrade above what it would be without any under- 

 ground heat. Hence, if the mean rate of solvent denudation was as 

 much as a thousand times what it is to-day, the result would have been 

 theaccomplishment of 100,000 years' denudation in the first hundred years. 

 If now we even lengthened the period of this excessive denudation, ten 

 times the correction would be no more than about 1 per cent. 



(b) For Direct Marine Denudation. — So far as the sea has directly 

 acted on the coasts, and on the sediments deposited in it, an error is in- 

 troduced calling for a subtractive correction on our estimate of geological 

 time ; in other words, upon our numerator, which is that part of the sodium 

 in the ocean supplied by sub-aerial denudation only. 



The total tide-swept area of the ocean is calculated by Sir J. Murray 

 and Professor Renard ('Challenger' Report) as 162x10' square kilo- 

 metres. The ' rainy ' area of the land is about 1 13 x IC' square kilometres. 

 The ratio of areas is 1 : 700. Hence, the assumption of a marine solvent 

 denudation having an intensity twenty times that progressing over an 

 equal area exposed to normal sub-atirial denudation would involve a sub- 

 tractive correction of rather under 3 per ct-nt. on geological time' The 

 solvent effects on sediments falling into the nearly, or quite, quiescent 

 waters beyond this zone are assumed to be small on the grounds of the 

 rapid flocculation and consolidation of marine sediments, as well as from 

 the fact that the very minute mineral particles of oceanic sediments show 

 little of such effects as would arise under conditions of sub-aei'ial denuda- 

 tion. They preserve, in fact, their soda in substantial excess of their 

 potash. There is in marine sediments generally almost complete absence 

 of the moi'e active acid and oxidising effects progressing in the soils. Nor 

 will the state of consolidation of marine sediments allow us to assume 

 anything like the enormous surface area, as much as 500 square metres 

 per litre, e.xposed within the soils. 



On these grounds it is assumed that the correction should not exceed 

 6 per cent., nor be less than 3 per cent. 



Our numerator was left at 14,694 x 10'- tonnes, and our denominator 

 at 15,.542 x 10^ tonnes. The resulting age of the earth would be 94 x 10" 

 years. A subtractive correction of 4 per cent, for marine denudation 

 leaves the geological age at 90 x 10'' years. Future extension of our 

 knowledge on the many points raised will, however, modify this number. 

 Thus, according to Professor De Lapparent's - more recent estimation of 



' See abstract of a paper read by the author before Section C, ' Some Experi- 

 ments on Denudation in Fresh and Salt Water.' 



^ Traits de Grolixjlc. tome i. p. 60. • 



