448 STUDIES FOR STUDENTS 



seas, and Chamberlin has oointed out^ that the effect of the greater 

 earth-movements has been to temporarily reduce to a minimum 

 these shallow submerged portions of the continent. Such move- 

 ments of continental uplift, or rather down-sinkings of the ocean 

 basins, seem, however, to carry up portions of the crust to heights 

 notably above the plane of isostatic equilibrium, from which they 

 gradually settle back toward equilibrium by virtue of the slow fluency 

 or quasi-fluency of the rocks. ^ At the same time, base-leveling is 

 proceeding from the margins of the continents, reconstructing new 

 coastal shelves, whose edges tend to become submerged both by the 

 slight filling of the sea with sediment and by the settling back of 

 the continental platforms. Assuming the truth of these general laws 

 of major earth-movements, subaerial delta- building encroaching upon 

 the shallow seas would attain greater importance as the erosion 

 verged toward maturity, since the amount of landwaste increases; 

 the streams, now being graded, carry it through to the shores, and 

 submerged continental platforms have had time to form. At the 

 same time, the deposits of arid interior basins and of Piedmont 

 slopes will diminish in importance, and finally become more or less 

 eroded. 



Finally, as the continent becomes topographically old, the moun- 

 tain slopes become subdued, the burden of the rivers lessens, they 

 can no longer build out extensive deltas against the seas, noteworthy 

 land deposits no longer form, and slight elevation of the ocean sur- 

 surface will cause it to widely transgress the base-leveled land. 



Turning to the marine deposition of mechanical sediments, they 

 should be observed to immediately increase in volume following con- 

 tinental uplift, several chief types being noted according to the nature 

 of the land and the movement of uplift. If from a near-by mountain 

 range, the sediments will be coarse in nature and formed through 

 rock disintegration more largely than through decomposition. If 

 from an older coastal plain, the marine deposits will comprise siliceous 

 residues from a previous period of erosion. If from a decomposed 

 regolith of a former near-by low-lying land, the sediments will be 



1 The Ulterior Basis of Time Divisions and the Classification of Geologic History, 

 Journal of Geology, -Vol. VI (1898), pp. 449-62. 



2 Ibid., p. 455. 



