GEOLOGICAL IMPORTANCE OF SEDIMENTATION 353 



and low tide the water could flow but a limited distance over tidal 

 flats, and in a more pluvial climate much of the region would doubt- 

 less be covered with fresh-water lakes and lagoons. Both salt and 

 fresh water, by wave - and current-action, would tend to differentiate 

 the water from the land. This conclusion is in conformity with what 

 is observed upon deltas facing the sea, the delta being normally divided 

 into a land surface occasionally covered by river floods and a sub- 

 aqueous portion perennially covered by the sea water, the two being 

 s'^parated by a relatively subordinate littoral zone, 



CONCLUSIONS ON GENERAL NATURE OF DELTA DEPOSITS 



A review of the deltas of the world at the present time shows that, 

 as a rule, the shallow basins marginal to the continents are filled with 

 iJluvial deposits, except in regions where the submergence or warping 

 is very recent, as in the case of the Baltic Sea and Hudson's Bay, or 

 where tidal scour tends to maintain an open estuary. 



The deltas of the larger rivers, where they have had a reasonable 

 geological time to form, customarily end in deep water beyond the 

 general limits of the coast-line. 



But the present is a time of continental extension and mountain- 

 building, though the late Tertiary was still more striking in this respect. 

 Therefore it may be reasonably concluded that at times of similar 

 topographic character in past geological history such epicontinental 

 seas as remained became, as a rule, largely filled, and usually rather 

 rapidly, by delta deposits, and shallow seas would thus give way to 

 alluvial plains. Even in times of partial continental submergence, 

 however, as in the upper Devonian and the Carboniferous, the 

 mountain-building then present would be expected by the accompany- 

 ing erosion to have given rise to extensive subaerial delta deposits, 

 which could, however, have only occupied the landward portions of 

 the broad epicontinental seas. Under such circumstances the pre- 

 ceding discussion has indicated that the beach might be a shifting 

 line, usually fluctuating about a certain limit, but sometimes trans- 

 gressing the alluvial delta plain, or again moving a shorter distance 

 seaward. The strata, as seen in vertical section, would be, under 

 these assumed conditions, to a considerable extent land deposits in 

 the region of thick sedimentation, largely marine where they thinned 



