354 STUDIES FOR STUDENTS 



out on the seaward side. The two kinds of formation would be inti- 

 mately interfingered at the contact — an interfingering which might 

 extend for tens, or occasionally for hundreds, of miles. 



The distinction, however, which it is wished to emphasize here is 

 that these deposits would be chiefly either land or marine, and only 

 subordinately littoral. The broad exposure of the land surface is 

 not a matter of between tides. Limited districts, as the Runn of 

 Cutch, might be regularly inundated by sea water during a certain 

 portion of the year, and other districts might be occasionally covered 

 by great storms; but more usually changes over considerable border 

 districts from land to sea would be a matter of centuries at least. In 

 interpreting such deposits, mud-cracks, rain-prints, foot-prints need 

 not necessarily, nor even usually, indicate exposures of tidal mud-flats, 

 as is usually assumed,^ but may equally well be interpreted as records 

 of a subaerial delta surface, regularly flooded by river inundations or 

 occasionally by the sea. Again, an occasional intercalation of strata 

 holding marine or estuarine fossils in a great series of mechanical 

 sediments is not evidence in itself that the entire formation is 

 marine or estuarine. On the other hand, an occasional occurrence 

 of fresh- water strata in an unfossiliferous formation is not evidence 

 in itself that the entire series is of continental origin. 



In times when the lands are topographically old, it is to be antici- 

 pated that land surfaces upon delta deposits will be at a minimum 

 since marine planation will not have been lessened in power; while, 

 on the other hand, the ability of the rivers to build out against the 

 seas, or to build up whenever subsidences occur, will have greatly 

 diminished. 



CONCLUSION ON PRESENT CONTINENTAL SEDIMENTATION 



It has been shown in the preceding pages that important conti- 

 nental deposits either now in process of formation, or so recently 

 made as still to exist as superficial formations, cover an appreciable 

 portion of the continents. 



Of these the interior formations of arid climates have been roughly 

 estimated to cover a tenth of the land surface of the globe, the pied- 



I The question of the origin and preserval of mud-cracks made on tidal flats is 

 considered later. 



