3i8 STUDIES FOR STUDENTS 



Besides these delta deposits, it appears that, taking the world as a 

 whole, interior basin deposits, both of desert and pluvial climates, are 

 now widely forming, frequently under favorable chances for preser- 

 vation, and that such deposits should also occur to an appreciable 

 extent in the geological record.^ 



Finally, it is concluded that the ratio of continental to marine sedi- 

 ments should have fluctuated widely through geological time. Fol- 

 lowing an epoch of continental uplift with mountain-making, the 

 deposits formed in interior basins should attain a maximum, especially 

 the deposits made under desert conditions. Accompanying this stage, 

 piedmont alluvial deposits would be formed, largely temporary in 

 character, since, upon the topography passing into the stages of matur- 

 ity, they tend to be removed by the rivers which laid them down. 



As the erosion verges toward maturity, subaerial delta-building, 

 encroaching upon the shallow seas, would attain greater importance, 

 since the amount of stream-dissection over the region of the head- 

 waters increases; the streams, now being graded, carry the sediment 

 through to the shores, and submerged continental platforms have had 

 time to form. The greatest proportion of subaerial alluvial deposi- 

 tion should take place after epochs of mountain-making which have 

 taken place without notable uplift of the continental platforms, as seems 

 to have occurred several times during the Paleozoic. Since in that case 

 a large portion of the river sediment is poured into more or less pro- 

 tected epicontinental seas, none of this portion reaches the deep ocean, 

 and much of it forms a subaerial delta, since the seas are shallow and 

 the wave-action weak. 



Eventually, as the continent becomes topographically old, the 

 mountain slopes become subdued, the burden of the rivers lessens 

 and becomes more largely rock matter in solution. The rivers can 

 no longer build out extensive deltas against the seas, and marine 

 planation, aided by a slight elevation of the ocean surface, may cause 



I While this article was being written, in December, 1905, a paper on "Torrential 

 Deposits and the Origin of Sandstones and Conglomerates" was read by title to the 

 Geological Society of America by Professor W. H. Hobbs, in which he argues for a 

 much larger proportion of subaerial arenaceous deposits in ancient strata than has 

 been recognized. This article the writer has not yet had the pleasure of reading. 

 The recent volumes of Chamberlin and SaHsbury, 1906, also place emphasis upon the 

 importance of subaerial river deposits, especially in Cenozoic times. 



