772 J. BAERPJLL MEASUREMENTS OF GEOLOGIC TIME 



areas of mountain systems, repeatedly worn low. A lesser portion came 

 from the reworking of older sediments from the gently up warped portions 

 of continental interiors. On these the erosion cycle was carried to such 

 completion during each period that when the waters spread over these 

 lands again the sediment was deposited on level surfaces of disconformity. 

 In these last stages of baseleveling the work of erosion must have become 

 extremely slack. 



The evidence of the sedimentary mantle points, then, to the conclusion 

 that the ocean level has stood lower in the Pliocene and Pleistocene and 

 the continents higher, with a more varied relief than they had previously 

 attained since the Precambrian. In figure 3 this conclusion is given 

 graphic expression. 



Curve A is the hypsographic curve, or curve of the present continental 

 relief, taken from the statistics given by Penck, as follows : 



Continental Relief 



Zones of altitude ' Relative areas 



Below sealevel 0.6 



to 200 meters 29.2 



200 to 500 meters 27. 1 



500 to 1,000 meters 19.0 



1,000 to 2,000 meters 16.4 



2,000 to 3,000 meters .3.6 



3,000 to 4,000 meters 2.1 



Above 4,000 meters 2.0 



100.0 



Our knoAvledge of the general nature of the hypsographic curve for the 

 Paleozoic periods of recurrent epeiric seas is drawn from the facts previ- 

 ously discussed. During the very widest spread of seas two-thirds of 

 jSTorth America is taken as covered and the mean depth of the epeiric seas 

 is assumed as 300 feet, or 90 meters. During the emergent stages be- 

 tween the Paleozoic floods, if the land is assumed to have become drained 

 to the present degree, the slopes of the land would nevertheless be so low 

 that two-thirds of its area is regarded as having been less than 600 feet 

 above sealevel. A hypsographic curve to fit these conditions is obtained 

 by dividing the ordinates of the present curve by four. This is shown as 

 B, figure 3, and it brings out vividly and graphically the great contrast in 

 land relief which the preceding discussion has indicated as existing be- 

 tween the present and certain past periods. 



In this diagram the sealevel has been kept for convenience as a constant 

 datum. It is probable, however, that the sealevel has sunk since the 



