THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. S7 



From the rate of the sun's loss of its stores of heat. (Tait.) 

 From other physical data. (Croll and others.) 

 (2) Geological. — {a) Calculations based upon the estimated 

 thickness of the geological deposits of the total series of strat- 

 ified rocks and the estimated rate of accumulation of deposits 

 along the shores of continents at the present time. (Hough- 

 ton, Dana, Croll, Wallace, Lyell, Humphreys and Abbott, etc.) 

 (/;) Calculations based upon rate of erosion since the 

 retreat of the glacial cover at the close of the Tertiary era. 

 (Dana, Lyell, Hall, Gilbert, Winchell, etc.); and general 

 estimates and sundry hypotheses as to the time since the 

 glacial age. (Geikie, McGee, Croll, Prestwich, Wright, 

 LeConte, and others.) 



Method of Computing Time from Thickness of Rocks. — The 

 elaborate report of Humphreys and Abbott on the " Physics 

 and Hydraulics of the Mississippi River" furnishes the kind of 

 evidence required for making the kind of calculations mentioned 

 under (2^) above — that based upon the rate of deposition, or 

 formation of deposits, at the mouth of rivers. The amount 

 of silt borne down and deposited by the Mississippi River 

 annually is estimated by Humphreys and Abbott to be equal 



to a mass with i square mile base and 241 feet deep, 



the earthy matter pushed along 27 " " 



or a total of sediment i mile square by 268 " " 



But upon Humphreys and Abbott's estimate, and distributing 

 the sedimentary deposit along the coast for a distance of 500 

 miles, and giving the strip 100 miles width (or spread it out 

 for 1000 miles, and make it 50 miles wide), assuming the area 

 of distribution of the product of erosion of the whole river 

 to be 50,000 square miles, — on such assumptions the deposit 

 in 6000 years would reach a depth of approximately 32 feet, 

 or 53 feet in 10,000 years; or, if we put it in round num- 

 bers, 50 feet in 10,000 years. The thickness of sediments 

 for the Devonian era is, according to Dana, 14,300 feet of 

 clastic sediments and 100 feet of limestone; estimating the 

 100 feet of limestone to be equivalent in time-ratio to 500 

 feet of ordinary fragmental sediment, we thus obtain in terms 

 of fragmental sediments a total of 14,800 feet. Reducing 



