102 SIXTEENTH REPORT. 



presented in the present series of papers was not available when this theory 

 was promulgated and will perhaps in tiliie lead to profitable reconsideration. 

 In any case it is apparent that if the permanence of the coast lines be ques- 

 tionable on the score of vertical movement the submarine border contours 

 are no less so. 



Marine encroachment by the action of waves and tides in tearing down 

 the land structure is doubtless a real force tending toward change of outline 

 and while its action is localized to the actual strand and marginal shallows 

 the resulting deposition is spread over a considerable area so that this factor 

 presumably changes the coastal outline faster than it does the hundred 

 fathom curve. The importance of this change for the present inquiry is 

 quite problematical, but it would seem, both a priori and in view of the 

 coastal parallelisms observed, that whatever its mean rate and whatever its 

 local variations this kind of marine encroachment has acted over long stretch- 

 es of coast with such a fair degree of uniformity that the major relations 

 have remained from the time of fracture to the present day. 



It seems reasonable to suppose that the edges of the continental shelf, 

 submerged to the depth of a hundred fathoms or thereabouts, formerly 

 represented the edges of the elevated plateaus left by the disruption of the 

 crust at the end of the Cretaceous period. 



If the Tertiary or even only the Pliocene, opened with the oceans very low 

 the processes of erosion must have been extremely active upon the heights 

 and most active of all at the edges. The edges of all the lands being precipi- 

 tous would naturally crumble and break down rapidly. Sea cliffs two or 

 three miles high could not long endure, the continental borders would speedily 

 be rounded down. And the material removed would pile up at the bottom. 

 Thus a more or less gentle slope would be established from the general ocean 

 bottom up to the ever-receding brink of the continental plateau. The 

 general land surface meanwhile, would be lowered, as it is always subject 

 to the erosive forces, but less rapidly than at the borders because of lower 

 gradient. 



I infer from evidence previously presented that at least one cause of the 

 Pleistocene Glacial Epoch was high altitude of land above sea level and that 

 such relative elevation was world wide in its occurrence. Also that its dura- 

 tion exceeded that of the glaciation which it helped to bring about. Cham- 

 berlin and Salisbury' speak of the Pliocene with the Pleistocene as together 

 constituting, except for the presence and effects of the ice, "a single period 

 of great land relief and oceanic restriction." 



This condition of high relief extended over the period when the great rivers 

 were cutting down deep gorges in their progress toward the reduction of the 

 general land level to that of the then low sea. During this time the mar- 

 ginal slopes, at first theoretically vertical, were being lowered to gentle 

 grades simultaneously about all lands and if at any time we could have 

 traced their crests upon a map we should doubtless have found them main- 

 taining fairly well their parallelism with the original fractured margins of 

 which they were but modifications All curves would be moved inland 

 concentricall}', the convexities of coast made smaller, the concavities larger. 



In view of the evidence indicative of such a former general state of high 

 relief of the lands, and considering the huge gorges which were carved by 

 the rivers flowing to the sea, some of which are still traceable upon the 

 submerged shelf, it would seem that there must have been ample time for the 



'Geology, vol. iii, 1906, p. .'^27. 



