THE CAMBRIAN PERIOD. 231 



pendent of internal forces at the time of its accomplishment. 1 As 

 will appear presently, superficial deformations are not excluded by 

 the general evidence against profound deformations. In the present 

 case we may first inquire how far gradation and allied agencies may 

 have been effective in bringing about the transgression of the sea over 

 a large part of the American continent in the later part of the Cam- 

 brian period. We may inquire next how far superficial deformations 

 may have cooperated with gradation in bringing about the transgres- 

 sion of the sea; and then, if these seem inadequate, we may resort 

 to the more hypothetical forces that reside in the deeper body of the 

 earth. 



(1) How far may gradation have been competent to bring about the 

 submergence of the continent? — Gradation is perpetual and inevi- 

 table where land and sea exist. The waves attack the land along its 

 borders, and the familiar agents of degradation reduce its surface. 

 The former is a direct cause of the encroachment of the sea upon the 

 land, and the latter an indirect one, since all sediments transported 

 from the land and deposited beneath the sea displace an equal volume 

 of water, and correspondingly, raise the sea surface. Small as this 

 rise must have been for any brief period, its effect was to cause the sea 

 to advance upon the land. The lowering of the land by degradation 

 at the same time, facilitated the advance. // continued long enough, 

 this cooperative process of shore-cutting around the borders of the 

 lands, of down-cutting over the whole surface, and of uplifting of sea- 

 level, must inevitably have caused the water to cover the whole con- 

 tinent and to spread deposits over all but the last remnants of it, pro- 

 vided there ivas no deformation of the body of the earth in the meantime. 

 This familiar process is illustrated in miniature in Vol. I, pp. 60-62, 

 where the lifting of the sea-level is neglected, because infinitesimal in 

 that case. Since this complex process is inevitable, it is well to assign 

 to it whatever it is competent to do consistently with the results of 

 other agencies known to have been in operation at the same time. 



It has been computed that if the earth, in its present condition, were 

 to remain without deformation for the requisite period, the erosion of 

 the continents and the transfer of the material to the sea would raise 

 the sea-level about 650 feet (Vol. I, p. 545). The last remnants of the 



1 For the general treatment of these two phases of deformation, see Vol. I, pp. 548- 

 589, and this Vol., pp. 119-132. 



