194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



the beginning of Cambrian time reaches the immense total of 259,000 

 feet. Of this great pile, 111,000 feet were deposited during Paleozoic 

 time, 86,000 during Mesozoic, and 61,000 during Cenozoic. The 

 aggregate thicknesses of strata on the other continents have never 

 been assembled, but it is believed that eventually the pile of strata for 

 the world will total 400,000 feet. To translate this thickness into 

 years, even approximately, is still an unsolved problem. A mean 

 rate of deposition that will hold for all strata can not be ascertained, 

 and the most that can be expected is a mean for each basin of depo- 

 sition. Even for limestones, which on the average take longer to 

 accumulate then either muds or sands, the rate can not be determined. 



It is therefore impossible in the light of present knowledge to check 

 by means of the sedimentary record the time determinations that are 

 based on radioactive disintegration. Schuchert therefore accepts the 

 estimate of 600,000,000 years, which is based largely but not wholly 

 on the radioactive evidence, as the best estimate we have for the 

 span of time since the beginning of the Cambrian, and shows that 

 the evidence of the strata can be harmonized with it. The data 

 based on radioactivity indicate that the ratio of Cenozoic time to 

 Mesozoic and Paleozoic is as 1:2:5; this ratio would require that 

 one foot of sandstone be deposited in 450 years, one foot of shale 

 in 900 years, and one foot of limestone in 2,250 years. These mean 

 rates of deposition are faster than any heretofore used in similar 

 estimates and are thought to be nearer the actual figures. 



Some evidence is beginning to appear that the rates at which 

 sediments were deposited in individual basins of sedimentation can 

 be determined, and these rates will afford valuable checks on the 

 determinations of geologic time that are based on atomic disinte- 

 gration. Such measurements become possible where the strata show 

 that they have been deposited by annual increments, each annual 

 increment consisting of a summer and a winter lamina. The couplet, 

 or annual layer, is called a varve. The difficulty in any given series 

 of strata is to prove beyond question that the layers are annual — are 

 really " varves " in fact. By counting the varves in the Green River 

 formation, Bradley has recently estimated that this formation was de- 

 posited in a period lasting between 5,000,000 and 8,000,000 years. As 

 the Green River formation appears to represent about one-third of 

 Eocene time, this estimated great length of the Eocene, which is 

 one of the shorter of the geologic time-periods, harmonizes well with 

 the evidence from rodioactivity. 



By counting the varves of the Bannisdale slates, which are 5,000 

 feet thick, Marr ^ calculated that these slates were deposited in 



= Marr, J. E., A Tossible Cbrononietric Scale for the Graptolite-bearing Strata. Falaeo- 

 biologlca, vol. 1, p. 161, 1928 ; see also Deposition of the Sedimentary Rocks, p. 1Q5, 1929. 



