820 J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



We may conclude consequently that because of times of non-deposition, 

 because of partial limitation of deposition by lack of subsidence, and 

 because of a less ratio of the Cordilleran sea to the total epeiric seas, 

 the time involved may have been five or ten times longer than the time 

 calculated by Walcott. Combining this with the reduction in the esti- 

 mates of rates previously discussed, the total time is multiplied by the 

 product of these corrections. The 16,300,000 years given by him as the 

 estimate for the time needed for the deposition of limestones is clearly 

 far too short. It might be multiplied by ten, twenty, or thirty to give 

 the duration of the whole Paleozoic. A twenty- or thirty-fold multiplica- 

 tion would accord more closely with the estimates given by the uranium 

 minerals, to be discussed later. 



Attention should next be given to the estimates of geologic time made 

 by Sollas, using the method advocated by Haughton — that the proper 

 measure of the length of the successive periods is given by the maximum 

 thickness of strata deposited in them. In 1895 Sollas estimated these 

 thicknesses for the post-Archean to aggregate 164,000 feet, in 1900 he 

 raised the sum to 265,000 feet, and in 1909^^ to 335,000 feet. The latter 

 figure is distributed among the geological eras as follows: Cenozoic, 

 63,000; Mesozoic, 69,000; Upper Paleozoic, 63,000; Lower Paleozoic, 

 58,000; Proterozoic, 82,000; Archeozoic, no estimate. 



These maximum thicknesses are deposited in geosynclines which may 

 be taken as deepest in the middle and of a triangular icross-section. He 

 takes a mean rate of denudation for all geological time of one foot in 

 2,400 years. To get the rate of deposition the drainage into the Gulf of 

 Mexico is used, the drainage area being taken" as 1,800,000 square miles 

 and the area of deposition as 100,000 to 180,000 square miles. Sollas 

 assumes consequently the mean deposition area for these thick formations 

 as one-tenth of the erosion area. This gives a mean rate of deposition 

 of one foot in 240 years. But in a triangular section the thickness of 

 the middle is twice the mean thickness. This would give a rate in the 

 region of maximum thickness of one foot in 120 years. But taking the 

 sides of the triangle as somewhat convex upward, Sollas obtains a rate 

 in the region of maximum subsidence of one foot in a hundred years. 

 This gives for the sedimentation of the Paleozoic 12,100,000 years, and 

 for all post-Archean time 33,500,000 years.^^ 



In his earlier estimates Sollas made no allowance for the time repre- 

 sented by unconformities, but in his latest work, giving apparently great- 



^"^ W. J. Sollas : Anniversary address of the president, Geol. Soc. London. Quart. Jour. 

 Geol. Soc, vol. 65, 1909, pp. Ixxxviii-cxxii. 



•'^ W. .7. Sollas : The age of the earth and other geological studies, 1905, pp. 36-38. 



