FHYTHMS IJC DENITDATION 773 



Paleozoic as much as 300 to 500 meters Avith respect to the continents, 

 and that the present average elevation of the lands, 730 meters, is in large 

 part due to that fact. Snch a sinking of the sea, due to increase in the 

 volnme of ocean basins, is suggested by the gain in height b}?^ every conti- 

 nent and the present approach to isostatic equilibrium for each continent 

 as a whole. In order to represent Paleozoic conditions, B and C should 

 then be elevated 300 to 500 meters with respect to A. This would bring 

 the two curves, A and B, of land relief closer together, and if A repre- 

 sents a fairly close approach to isostasy, would minimize the isostatic 

 strain which is implied by the flatness of the Paleozoic lands and permit 

 more readily the control of their level by sealevel rather than by isostatic 

 rejuvenation. The change from the types of Paleozoic to Cenozoic conti- 

 nental reliefs has, then, been accomplished by an increase in the volume 

 of the ocean basins attended by a warping down of the negative areas of 

 the continents several hundred meters and a warping up of the positive 

 areas to a somewhat greater amount. This tendency to differential warp- 

 ing existed in the Paleozoic, but a marked relief was not attained because 

 erosion of the positive areas led to leveling by sedimentation on the nega- 

 tive areas. The tendency to rise of sealevel during a single cycle of con- 

 tinental erosion — a geologic period — may be due most largely to the dis- 

 placement of sea-water by sediment ; the rise during the Paleozoic era may 

 better be ascribed to the addition of juvenile waters. This latter cause 

 has continued in operation in later time, but has been more than offset by 

 an increase in volume of ocean basins and consequent lowering of the 

 mean sealevel, due presumably to continental fragmentation. 



CONTRAST OF PRE^WNT AND PAST RATES OF DENUDATION 



The Precambrian ages were of immense duration and broken^up into 

 eras by profound revolutions. Sediments were accmnulated in great 

 thickness in geosynclines, but over the continents as a whole recurrent 

 erosion exceeded sedimentation. The result was that at the opening of 

 the Paleozoic metamorphic and igneous rocks of the character of those of 

 the Canadian shield constituted the dominant surface formations. To 

 have exposed such rocks the continents must have been broadly elevated 

 and deeply eroded. During pulsatory uplift the rate of denudation doubt- 

 less rose at times to the present high rate, and may even have exceeded it, 

 since vegetation could not then have been an efficient binder for the soil. 

 Following such periods of erosion the lands were low and may have rested 

 in that attitude of old age for vast periods of time. In illustration, it 

 appears that following the last great Precambrian revolution the surface, 

 although of low relief, still lay generally above sealevel, the whole of this 



