BUFFALO SOCIETY OF NATURAL SCIENCES 79 



Fluviatile Deposits and Faunas. The importance of sedi- 

 ments containing river faunas has not heretofore been realized, nor 

 have such sediments and their characteristics been dwelt upon by 

 most geologists. Grabau has been the staunchest advocate of the 

 fluviatile origin of many deposits both in this country and Europe, 

 but has usually stood alone in his interpretations. In his paper on 

 "Early Palaeozoic Delta Deposits of North America" he has described 

 in great detail a large number of delta deposits occurring in the Ordo- 

 vic'c and Siluric and has shown what are the characteristics physical 

 and faunal of such deposits. Barrell has likewise made a number of 

 contributions to the study of fossil delta deposits with especial em- 

 phasis on their physical characteristics, and on the climatic factors 

 control 1 ing sedimentation. 



It is a matter of difficulty to determine much about rivers of older 

 geological periods, because the river channels are seldom preserved, 

 especially in the Palaeozoic, and when found are visible, usually only 

 in section and cannot be traced along the surface. Flood plain and 

 delta deposits are almost the only records of their presence left by 

 ancient rivers. It is not to be expected, however, that such deposits 

 will be without fossils any more than similar deposits today are. 

 Rivers carry large amounts of detritus varying in grain from fine 

 muds, a fraction of a millimeter in diameter, to bowlders often sev- 

 eral feet across, though these coarser elements are more likely to be 

 carried by torrential or mountain streams than in the larger rivers. 

 In a pluvial climate this load is brought into lakes or to the ocean and 

 there deposited; in an arid climate it is spread out on interior plains 

 or in basins in the form of alluvial fans or dry deltas. Since the 

 lithological characteristics of deltas and flood plains are often of 

 great assistance in the recognition of fossil deposits of this type, it 

 may not be amiss to say a few words about them here. 



The sediments spread out by a river in its lower reaches are of 

 two types: (a) those which form directly at the mouth and are spread 

 out in front of it into the sea, and (b) those which are spread out lat- 

 erally either over the subaerial portion of the delta or along the flood- 

 plain and over the neighboring lowlands throughout the lov/er por- 

 tions of the river. These are the fine mud deposits of which we see 

 such splendid examples in the case of the Nile and Mississippi deltas. 

 The deposits in the Nile delta are thus described: "At low water 

 these are visible in the steep banks which then rise 8 to 10 meters 

 above water level. The hardened Nile mud forms a series of hori- 



