GEOLOGIC TIME. 307 



be cap.able of moditication or amendment. lUit we must also remember 

 that the geological record constitutes a voluminous body of evidence 

 regarding the earth's history which can not be ignored, and must be 

 explained in accordance with ascertained natural laws. If the conclu- 

 sions derived from the most careful study of this record can not be recon- 

 ciled with those drawn from physical considerations, it is surely not too 

 much to ask that the latter should be also revised. It has been well 

 said that the mathematical mill is anadmirablepieceof machinery, but 

 that the value of Avhat it jaelds depends upon the quality of what is put 

 into it. That there nuist be some flaw in the physical argument, I can, 

 for my own part, hardly doubt, thongh I do not pretend to be able to 

 say where it is to be found. Some assumption, it seems to me, has 

 been made, or some consideration has been left out of sight, which will 

 eventually be seen to vitiate the conclusions, and which, when duly 

 taken into account, will allow time enough for any reasonable interpre- 

 tation of the geological record."* 



Of the rate of denudation and deposition he says : 



" The rate of deposition of new sedimentary formations, over an area 

 of sea tloor equivalent to that which has yielded the sediment, may 

 vary from 1 foot in 730 years to 1 foot in 0,800 years. If now we take 

 these results and apply them as measures of the length of time reqnired 

 for the deposition of the various sedimentary masses that form the 

 onter part of the earth's crust, we obtain some indication of the dura- 

 tion of geological history. On a reasonable computation these strati- 

 fied masses, where most fnlly developed, attain a united thickness of 

 not less than 100,000 feet. If they were all laid down at the most rapid 

 recorded rate of denudation, they would require a period of 73,000,000 

 years for their completion. If they were laid down at the slowest rate 

 they would demand a period of not less than 080,000,000." t 



Reade. — Mr. T. Millard Reade has been a large contributor to the litera- 

 ture of geologic time, both directly and indirectly. His most recent con- 

 clusion is that there appears to be a consensus of oi^inion that 1 foot 

 in 3,000 years is a fair estimate of the mean rate of such erosion over 

 all land areas throughout all geologic time. The calculation that has 

 elapsed since the beginning of Cambrian time, on this basis,*is stated 

 as follows: 



" The mean area of denudation throughout post-Archaean times being 

 taken as one-third the entire land areas of the globe, the bullc of the 

 post-Archean rocks being ex])ressed by the land area of the globe 2 

 miles thick, and the rate of denudation 1 foot in 3,000 years, the time of 

 accumulation will be 5,280 x 2 x 3,000 x 3 ^ ()r),()40,000!; The time that 

 has elapsed since the commencement of the Cambrian is, therefore, in 

 round figures, 95,000,000 years." | 



S])eaking of Sir Archibald Geikie's conclusion that the earth's age, 

 geologically speaking, nmst be somewhere between 100,000,000 and 

 600,000,000 years, he says : 



"This is a large margin, no doubt, but it is an important thing to 



* Presidential Address. Report Sixty-second Meeting Brit. Assoc. Adv. Sci., 1892, 

 pp. 19, 20. (Also Smithsonian Report for 1892, pp. 125, 126.) 

 iLoc. cit., p. 21.— Sm. Report, 1892, p. 127. 

 t" Measurement of geological time." GeoL Mag., 1893, vol. x, pp 99, 100. 



