Page:The Habitat of the Eurypterida.djvu/85

. The importance of sediments 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 Palæozoic Delta Deposits of North America" he has described in great detail a large number of delta deposits occurring in the Ordovicic 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 emphasis on their physical characteristics, and on the climatic factors controlling 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 Palæozoic, 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 several 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 laterally either over the subaërial portion of the delta or along the flood-plain and over the neighboring lowlands throughout the lower portions 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