(SI 12 J. liAKREJ.L MEASUREMENTS OF GEOLOGIC TIME 



unities are reduced to a miiiimmn. Eecognizing that they still exist, it 

 will be of value for comparison of the present conditions with the past to 

 take the maximum known thickness for each era and divide it by the 

 length of the era in years in order to get the comparative times repre- 

 sented by one foot of sediment. For this purpose the minimum ages 

 given by uranium minerals will be used, as more probably of the proper 

 magnitude. The tabulation follows : 



Geologic interval Time interval Maximum thickness Time for one foot 



Years ' Feet Years 



Pleistocene 1,500,000 4,000 875 



Oenozoic 55,000,000 63,000 875 



Mesozoic 135,000,000 84,000 1,600 



Neopaleozoic 200,000,000 78,000 2,600 



Eopaleozoic 160,000,000 43,000 3,700 



It is seen that the rate of accumulation of the geosynclinal deposits 

 appears to grow progressively slower with remoteness in time. There are 

 several factors contributing to this effect in addition to the discontinuity 

 which has been emphasized. 



First. It has been shown that the Pleistocene has been a period marked 

 l)y grea^ orogenic movements and by high continental uplift, accom- 

 plished in a pulsatory manner. The Pleistocene, including the Eecent, 

 constitutes the culmination of a period of revolution, and the rates of 

 erosion and sedimentation are both in consequence abnormally high. 



Second. The stratigraphic record of the Cenozoic is fairly complete, 

 since various deposits, largely continental, made above sealevel and geo- 

 logically temporar}^, fill in the gaps represented by unconformities in 

 other localities. Such a complete record could not be expected for an 

 older era. 



Third. As to the relative rates for the several eras, although the mean 

 rate of erosion and sedimentation of the Tertiary should show less than 

 the Pleistocene, the Tertiary, nevertheless, consists of two periods of revo- 

 lution and its rate is naturally higher than that of the Mesozoic as a 

 whole. There were long times of quiet and spreading seas in the Meso- 

 zoic when the surface processes must have sunk to a low rate. The blanks 

 left by unconformities, furthermore, begin to play a larger part. 



Fourth. In the Neopaleozoic the continents stood lower, as shown by a 

 more prevailing spread of epeiric seas as compared to the Mesozoic. A 

 low attitude of the continents reduces the area which is subjected to ero- 

 sion and, even if the rate for the remaining land areas is high, the total 

 volume of sediments per year for the whole continent is low. The sedi- 

 ments derived from a small land area are spread over a wide shallow sea 



