60 GEOLOGICAL BIOLOGY. 



Ganges 2358 years. 



Mississippi 6000 " 



Hoang-Ho 1464 " 



Yangtse-Kiang 2700 " 



Rhone 1528 " 



Danube 6846 " 



Po 729 " 



found the mean rate to be 3090 " 



From this table he concluded that " atmospheric agencies 

 are capable, at present, of lowering the land-surfaces at the 

 rate of I foot per 3000 years; but since the sea bottoms 

 are to the land surfaces in the proportion of 145 to 52, the 

 rate at which (under present circumstances) the sea bottoms 

 are silted up, that is to say, the present rate of formation of 

 strata, is I foot in 8616 years. If we admit (which I am 

 by no means willing to do) that the manufacture of strata in 

 geological times proceeded at ten times this rate, or at the 

 rate of I foot for every 861.6 years, we have for the whole 

 duration of geological time, down to the Miocene Tertiary 

 epoch, 861.6 X 177,200 = 152,675,000 years. The coeffi- 

 cient 177,200 is the total number of feet of maximum thick- 

 ness of all the known stratified rocks." 



In this same paper Houghton expresses, in concise terms, 

 the following conclusion, viz. : " The proper relative measure 

 of geological periods is the maximum thickness of the strata 

 formed during these periods." 



If this sediment be distributed over a strip 30 miles wide 

 and 100,000 miles long the estimated coast border of depo- 

 sition, amounting to an area of 3,000,000 square miles, or 

 ly 1 ^ of the land area, on this area the accumulation will be 

 nineteen times as fast as estimated for the whole area, or I 

 foot in about 158 years. Assuming this to be a more cor- 

 rect estimate of the actual depositing-ground, Wallace, taking 

 Houghton's estimate of 177,200 for the total maximum 

 thickness of the stratified rocks, gets for the time-period of 

 the deposition of their thickness, approximately, 28,000,000 

 of years. 



(3) The proportion between fragmental sediments and 



