FOSSILS OF DELTAS 405 



Beyond the delta, moreover, there must be a corresponding purely marine 

 series of deposits, formed in the open sea while the delta was building 

 along the coast. 



FOSSILS OF DELTAS 



The fossils of subaerial or dry deltas, as already noted, are chiefly of 

 terrestrial types, though on very large deltas, swamps, or even lakes may 

 come into temporary existence, and so paludal and lacustrine organisms 

 may occasionally become entombed. River organisms are apt to form an 

 important part of the organic content of such dry deltas, but marine 

 organisms are wholly wanting, except in the cases where the margins of 

 the delta dip into the sea, or where the sea momentarily trangresses across 

 the delta. 



Seashore deltas, on the other hand, are likely to contain many remains 

 of marine organisms, but these are generally distributed in layers alter- 

 nating with others, in which remains of marine organisms are wanting 

 and where often terrestrial organisms take their place. Remains of land 

 and river organisms constitute a characteristic part of the typical sea- 

 shore deltas, and indeed it might be considered that a delta of this type 

 free from such remains is an anomaly and requires special explanation. 

 Among the common type of terrestrial fossils of deltas are plants, espe- 

 cially the trunks of trees, which are brought down by the rivers and 

 stranded in the delta. But river animals and plants must also be repre- 

 sented. Aquatic plants are often prolific in spores, especially the Rhizo- 

 carps, and such bodies might be expected in abundance. The spore cases 

 of Chara would also be subject to burial in the delta deposits. River 

 animals would be chiefly represented by dead parts, such as the exoskele- 

 tons of crustaceans, etcetera. In general, when a group of organisms is 

 practically restricted to deposits of the delta type, it is reasonably safe to 

 assume that such organisms are of fluviatile or terrestrial rather than of 

 marine habitat. 



OVERLAP RELATIONS OF DELTAS 



The normal type of overlap of the alluvial fan, formed entirely on dry 

 land, is by radial extension of the later layers, so that they transgress the 

 margins of the preceding deposit, and each layer in turn comes to rest 

 directly on the old surface on which the fan is building. Such overlap 

 will be most marked around the periphery of the fan, the overlap thus 

 being away from the source of supply. A head ward overlap, or the ex- 

 tension of the higher layers up the stream channels supplying the ma- 

 terial, may be brought about by the change in grade which results with 



