838 J. BARRELL- — MEASUREMENTS OF GEOLOGIC TIME 



according to Becker, that the mean area of N'orth America above water 

 has been eight- tenths of the present area. It may, however, have been 

 more or less. As Schuchert recognizes, his maps minimize the limits of 

 the epeiric seas. At times they probably extended far beyond the ascer- 

 tainable bounds, since erosion of the following periods would remove the 

 thin outlying deposits; but in the intervening stages, as Schuchert was 

 one of the first to emphasize, the shallow seas were practically withdrawn 

 from the continent. 



The rate of erosion, however, does not turn so much on the area of 

 land as on its elevation. During most of the Paleozoic and Mesozoic the 

 continents stood notably low with respect to sealevel. It was particularly 

 the mountain axes and plateaus which suffered erosion. The great Cana- 

 dian Shield of older rocks stood at a low elevation and suffered but little 

 erosion between the Proterozoic and Cenozoic — a fact first emphasized by 

 Bell.^^ The same relations were true of other continents. Although deep 

 geosynclines were filled with sediments and imply the destruction of great 

 mountain ranges, their volume was small as compared to the broad areas 

 of the continents now well above baselevel and suffering rapid denuda- 

 tion. Erosion could be locally rapid, but the bulk of erosion of igneous 

 rock must have been relatively small. 



Since the beginning of the Ordovician, furthermore, the actual area of 

 igneous rocks has not diminished after an exponential law. It appears to 

 have varied considerably with the broad covering and uncovering by sedi- 

 mentary blankets, and to have been greatly increased at times by igneous 

 extrusions. The Cenozoic has been a period of notable igneous activity, 

 and in North America the erosion of the Cordilleran area is largely from 

 Cenozoic extrusives and Mesozoic intrusives. The Ordovician witnessed 

 a very wide expansion of epeiric seas and the erosion of igneous rocks 

 was reduced to a minimum from that time forward to the Appalachian 

 revolution. In view of these actual geological conditions the refinements 

 of calculation introduced by Becker seem useless and lend only a false 

 security to the results. Joly's assumption of uniform rate of denudation 

 comes nearer to the truth ; but, in view of the existence of cycles of erosion 

 through geological time, even Joly's figure may have to be multiplied 

 many times. 



ESTIMATES OF TIME BASED ON LOSS OF PRIMAL HEAT 



Before the discovery, at the opening of the twentieth century, of the 

 wonderful field of radioactivity, it seemed to physicists that the heat lost 



8' Robert BeU : Pre-Paleozoic decay of crystalline rocks north of Lake Huron. Bull, 

 (lool. Soc. Am., vol. .5, 1804, pp. 857-.H0G. 



