326 T. C. CHAM BERLIN 



channel; (2) that done in the more hmited tract of the stream 

 channel itself. It is essential to note the normal differences 

 between these two forms of deposit and their working relations 

 to one another. 



(i) The flood-plain deposits are formed of the residue of what- 

 ever grows upon the flood-plain, or may fall upon it in any way, or 

 may be blown upon it by the winds, or may be mired in it when 

 wet and soft, together with whatever may be washed upon it by 

 the stream at the times of freshet or flood. When first spread 

 upon the bottom, the deposits of freshet and flood stages are 

 usually stratified, but they are quite sure to suffer serious disrup- 

 tion by the growth of plants, the tread of animals, and various 

 other surface agencies, so that they very commonly lose their 

 original structure. On bottoms of low gradient in a sandy tract 

 flood-plain deposits are thus likely to be a heterogeneous mixture 

 of vegetal and earthy muck combined with more or less sand, and 

 to embrace as incidentals the hard parts of whatever lives and 

 leaves its relics on the flat surface. 



(2) The channel deposits, on the other hand, are usually 

 formed mainly of the normal products carried by the stream along 

 its bottom, notably sand and pebbles, but. these are quite sure to 

 be more or less mixed with derivatives from the flood-plain deposits 

 that form the banks of the stream. More or less constantly these 

 muck banks are being undercut and caused to slide into the chan- 

 nel where they are then more or less reworked by the stream. 

 Usually the muck masses are gradually disintegrated and washed 

 onward by the stream, but they may be buried bodily in its sands, 

 thus carrying into these sands whatever the muck masses happen 

 to contain. This content is likely to retain whatever condition 

 it happened to have when in the muck bank. 



In addition to these normal processes the stream, at appro- 

 priate points here and there, is also engaged in the work of scour- 

 and-fill. This accidents the results and adds its special features 

 to the products of the more uniform channel work. 



Now this tripartite work is pre-eminently a mixing process; 

 at the same time it is an assorting process. Each of these two 

 processes is likely to be carried to varying degrees in the mul- 



