Age of the Earth. 369 



A discussion of the time-allowance for these early uncon- 

 formities would lead us too far into speculation. It may be 

 observed, however, as regards the evidence for prolonged 

 periods of denudation deduced from regional base-levelling, 

 that the instability of the early crust must again be kept in 

 mind. It is probable that the Algonkian mountains were not 

 of the dimensions of those of later periods and that, therefore, 

 they were at once more rapidly formed and more rapidly 

 removed. Van Hise and Leith suggest that the unconformities 

 may represent as much sediment again as now remains to 

 observation. This, of course, can only be matter of opinion ; 

 and I have as far as possible endeavoured to exclude what is 

 purely matter of opinion from this review of the subject. It 

 would seem, however, that BoHas' estimate of 82,000 feet 

 of sediment includes such an allowance as appears possible 

 to Van Hise and Leith. 



Taking all into account — and much has been omitted which 

 might be said upon the subject — it does not appear that 

 Professor Sottas' compilation of the stratigraphical column 

 need be seriously disturbed. If we double the estimate for 

 the Jurassic we at least tend to reduce the possibility of error 

 of deficiency in the thickness assigned to this system. This 

 brings the column up to, say, 345,000 feet. 



What now, finally, is the time value of this enormous total ? 

 Unfortunately the average rate of collection is a very inde- 

 terminate quantity. We are, I believe, at liberty to assume 

 that the rate of deposition and sinking was anything from, 

 say, one foot to but a few inches in a century. A rate 

 of accumulation of four inches in a century interprets the 

 geological column us indicating 103 millions of years. Three 

 inches gives us 148 millions. The order of the time-value is 

 probably indicated in these figures. 



It is important to note that the facts of solvent denudation 

 place a quite definite limit on the amount of sediments which 

 have been formed during geological time. The sodium 

 which has reached the ocean has originated in the conversion 

 of igneous into sedimentary rocks. It is easy to calculate 

 from the composition of a generalized igneous and a gene- 

 ralized sedimentary rock and from the quantity of sodium in 

 the ocean that the denudation of about 84 million cubic miles 

 of igneous rock, producing about 60 million cubic miles of 

 sediment, accounts for the sodium in the ocean. Such a 

 quantity of sedimentary rock would, if all was now on the 

 land, cover the present land area (55 million square miles) to 

 a depth of a little over one mile. As it can be shown that 

 somewhat less than a third of the sediments have been preci- 



