GEOLOGICAL IMPORTANCE OF SEDIMENTATION 355 



mont waste slopes of several continents to cover an area of the same 

 order of magnitude as that of the great mountains from which they 

 come. Interior basins of pluvial climates are seen to be largely filled by 

 fiuviatile deposits; again, where the land meets the sea sediment is 

 deposited as deltas, forming fiuviatile deposits with bottoms far below 

 the level of the sea. These have been observed to be of varying 

 importance in different continents, and no estimate of their aggregate 

 areal extent has been given. Still, it would seem to be in the neighbor- 

 hood of the truth to place the subaerial deposits of piedmont waste, 

 of continental basins, and of deltas as covering a tenth of the emerged 

 continental surfaces. Adding this to the estimate of the deposits of arid 

 climates would give a fifth of the land surface as mantled by conti- 

 nental formations. From the generalized profile showing the relative 

 areas of the earth's crust at different heights given by Penck,'' and 

 also by Gilbert, ^ from Murray's figures it is seen that one-fifth of the 

 land surface is elevated more than 1,200 meters above the sea> The 

 more elevated portions of the crust suffer, on the whole, the most 

 severe and rapid erosion. Consequently, a general idea of the locali- 

 zation and rapidity of erosion upon the land may be gained by stating 

 that at present one-fifth is subject to extremely rapid erosion, one- 

 fifth to rapid erosion, one-fifth to moderate erosion, one-fifth to slight 

 erosion, and one-fifth, either now or in recent geological times, to 

 sedimentation. Not all of the latter would be permanently pre- 

 served by the continued action of the forces which have led 

 to their accumulation. Such superficial formations as those deposited 

 upon slightly warped slopes previously graded, as the Lafayette forma- 

 tion of the eastern United States is presumed to be, or the thin deposits 

 of ancient deserts, are especially liable to be destroyed, so that the geo- 

 logical record of former ages should show a far less proportion of thin 

 and superficial land deposits. Basin deposits of either desert or plu- 

 vial climates and delta deposits possess, however, indefinite thickness, 

 and the chances of indefinitely long preservation of at least the lower 

 portions is nearly as good as in the case of the deposits upon the floors 

 of epicontinental seas. As, however, changes in geological activities 



1 Morphologic der Erdoberfldche, Vol. I (1894), p. 136. 



2 "Continental Problems," Bulletin of the Geological Society of America, Vol. IV 

 (1892), p. 180. 



