FRESH-WATER STREAMS. 649 



the Mississippi pushes along into the Gulf large quantities of earthy 

 matter ; and, from observations made by them, they estimate the an- 

 nual amount thus contributed to the Gulf to be about 750,000,000 

 cubic feet, — which would cover a square mile twenty-seven feet deep ; 

 and this,added to the 241 feet above, makes the total 268 feet. 



The quantity of wood brought down by some American rivers is 

 very great. The well-known natural " raft," obstructing Red River, 

 had a length, in 1854, of thirteen miles, and was increasing at the 

 rate of one and a half to two miles a year, from the annual accessions. 

 The lower end, which was then fifty -three miles above Shreveport, 

 had been gradually moving up stream, from the decay of the logs, and 

 formerly was at Natchitoches, if not still farther down the stream. 

 Both this stream and others carry great numbers of logs to the delta. 



3. Distribution of transported Material. 



1. Alluvial Formations in River-valleys. — Alluvial formations cover 

 usually a broad area, on one or both sides of a river. Thej r are in 

 general the basis of the flood-plain ; and the features of this plain, as 

 already described, are the exterior characteristics of the alluvium. 

 They are made from the material brought down by the stream, espe- 

 cially during freshets, and consist of earth and clay, sometimes thinly 

 laminated, with some beds of pebbles, and occasionally stones. These 

 coarser beds are most abundant along the upper portions of the 

 stream ; while, toward the mouth, — particularly in the case of large 

 rivers, — the material may be wholly a fine silt. When the floods are 

 very great and of long continuance, as during the melting of the 

 glacier in the Champlain period, the finer depositions may have no 

 distinct bedding, where the waters flow quietly, or, on the contrary, in 

 case of a violent plunging flow, the flow-and-plunge structure described 

 on page 83. 



The material, whether coarse or fine, is, in general, simply pulverized rock — the 

 rocks of earlier time ground to powder, by the attrition undergone through the moving 

 waters. So that a mud often consists of the very same minerals, and in the same pro- 

 portions, as the granyte or gneiss from which it was derived; in such a case, the feld- 

 spar is present as feldspar, this being proved by the presence of potash or soda, which 

 ingredients are lost when feldspar undergoes decomposition. Most of the shales and 

 slates of the world are made from muds or clays of this kind. But, in other cases, the 

 feldspar has undergone more or less complete decomposition; and then the muds or 

 clays have a different constitution, the alkalies being absent. 



Again, since, in the flow of water, softer materials are worn out, and also the lighter 

 borne on to stiller regions, quartz sands are often left by themselves, and the finer silt 

 carried to make deposits of its own : and thus again the deposits are made to differ in 

 constitution from the rocks whence they are derived. The facts here stated are true 

 also of glacial and marine depositions. 



Logs and leaves are in some cases distributed through alluvial de- 



