58 GEOLOGICAL BIOLOGY. 



this 14,800 feet of thickness of sedimentary deposits into time- 

 equivalent, on the basis of the above rate of formation of sedi- 

 ments, we have 2,960,000 years for the duration of the 

 Devonian era. If now we assume the Devonian to be ap- 

 proximately \o<fc of the whole time-duration from the base of 

 the Cambrian to the present, the total time-duration would be 

 29,600,000, which is a little over one half the estimate sug- 

 gested by Dana, viz., 48,000,000 years since the beginning 

 of the Paleozoic time — Paleozoic 36,000,000, Mesozoic 

 9,000,000, and Cenozoic 3,000,000.* 



Forshay's estimate makes the amount of annual deposit 

 964 instead of 268 feet on a base i mile square in i year's 

 time, which is about four times as rapid accumulation as the 

 estimate of Humphreys and Abbott, and the effect upon time- 

 duration expressed by rock-thickness would be to reduce the 

 time one fourth, making the Devonian 740,000 instead of 

 2,960,000 years long. This would bring the age of the earth, 

 as a solid globe, nearer to the estimate of Clarence King 

 (24,000,000 years), to which Lord Kelvin gave approval as 

 lately as March, iSgS.f 



Errors arising from Estimated Values in the Computations. — 

 According to this estimate we notice that there are several 

 important data which are assumed, and not observed or known. 



(i) The thickness of the deposits themselves. Forma- 

 tions, as may be noticed, vary greatly in thickness for even 

 the few localities or regions of America in which they have 

 been studied. We find that the maximum thickness of the 

 North American Paleozoic series is given as 55,000 feet, the 

 general thickness of these deposits in the Appalachian region is 

 40,000, and in the interior of the continent it varies from 

 6000 to 3500. Since this estimate was made, Walcott has 

 claimed for the Cambrian 7000 feet of fragmental rocks and 

 200 of limestones; in the Rocky Mountain province 10,000 

 feet of fragmental and 6000 feet of limestones, which, reduced 

 to time-ratios (-f- for limestone), gives, instead of (7000 -j- 

 1000 =) 8000, (10,000 4- 30,000 =) 40,000, or five times the 



* See " Manual of Geology," 3d edition, p. 591. 

 •f- See Nature, vol. LI. pp. 43S-450. 



