INTRODUCTION 



61 



Modes of Estimating Ccenozoic Time 



From the rocks themselves there are several modes of calculation: 

 1. Total thickness of the formations composing the so-called sedimentary 

 rocks, compared with the average rate of accumulation, deposition, and 

 sedimentation observable to-day. 2. Denudation and erosion, the counter 

 processes, or the wearing away of elevated surfaces by the action of water 

 and wind, snow, ice, and frost. Estimates of former 

 heights of mountains, etc., antl of the length of time 

 during which these erosive agencies have been at work. 

 3. Chemical content of the sea, based on the assump- 

 tion that all the salts and mineral elements of the sea 

 are derived by solution from the soil. 4. Procession 

 and recession of the glaciers as a means of estimating 

 Pleistocene or Quaternary time by comparison of past 

 with present advances and retreats of glacial masses. 



Obstacles confront every mode of making these 

 comparisons of past and present processes. In estimat- 

 ing past rates of accumulation by those observed in the 

 deltas or mouths of existing rivers, the disturbing and 

 unnatural influence of man must be considered. The 

 modern delta accumulations of the Mississippi, the Po, 

 the Danube, the Tigris, the Euphrates, and the Ganges 

 are probal^ly unnaturally rapid because the soil of the 

 drainage basins from which these deltas are formed has 

 been disturbed by the unnatural erosion hastened by 

 human cultivation. In the case of the Tigris and ^ ^ 



T-11 1. n r c fc-1 <• r\G. 15. — Duration 



Euphrates, the makmg of from forty to fifty miles of of the Age of Mam- 

 new land in the Persian Gulf, so that ancient seaports 'V""'^ ''^ , "\^'i«^'"*:^l ^^ 



,. • , , the evolution of the 



of tour or nve thousand years ago are now tar inland, teeth of the horse. Su- 

 is very largely due indirectly to human agency, namely, pp"or grinding teeth 



,,. ,. -, n 1 .,, "^'of Eohippus (below) , 



to the destruction ot the forests, the unrestricted brows- and of Equus (above), 



drawn to scale and 

 showing the great in- 

 crease in Complexity as 

 well as in length (A) 

 and width (B). 



The Mississippi, 



ing of sheep and goats, and the consequent rapid de- 

 nudation of the soil. 



Deposition or accumulation. — An outline of the 

 methods employed to calculate rates of deposition may 

 be found in Williams's "Geological Biology" (1895). 

 according to the calculations of Humphreys and Abbot, brings down every 

 year sediment equivalent in amount to a mass 268 feet deep and one 

 square mile in extent. Assuming the area of distribution to be 50,000 

 square miles, the deposit would reach a depth of 50 feet in about 10,000 

 years, or one foot in 200 years. Forshay estimated the Mississippi ac- 

 cumulation as four times as rapid, or at the rate of one foot in 50 years. 

 The most precisely measured flood plain in the world is that on either 



