118 FLUVIATILE DEPOSITS OF THE NILE. [Ch. X. 



— As a general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 

 4, fig. 106) are severally made up of coarse materials in their lower 

 portions, and of fine silt or loam in their upper parts. For rivers are 

 constantly shifting their position in the valley-plain, encroaching 

 gradually on one bank, near which there is deep water, and deserting 

 the other or opposite side, where the channel is growing shallower, 

 being destined eventually to be converted into land. Where the cur- 

 rent runs strongest, coarse gravel is swept along, and where its veloci- 

 ty is slackened, first sand, and then only the finest mud, is thrown 

 down. A thin film of this fine sediment is spread, during floods, over 

 a wide area, on one, or sometimes on both sides, of the main stream, 

 often reaching as far as the base of the bluffs or higher grounds which 

 bound the valley. Of such a description are the well-known annual 

 deposits of the Nile, to which Egypt owes its fertility. So thin are 

 they, that the aggregate amount accumulated in a century is said 

 rarely to exceed five inches, although in the course of thousands of 

 years it has attained a vast thickness, the bottom not having been 

 reached by borings extending to a depth of 80 feet towards the cen- 

 tral parts of the valley. Everywhere it consists of the same 

 homogeneous mud, destitute of stratification — the only signs of suc- 

 cessive accumulation being where the Nile has silted up its channel, 

 or where the blown sands of the Libyan desert have invaded the plain, 

 and given rise to alternate layers of sand and mud. 



The general absence of lamination in the loam of the Egyptian 

 river-plain is probably owing to the thinness of the layer thrown 

 down in a single year, and to its being exposed "for eight months to 

 drying winds, or the rays of a hot sun. Parts of it are often swept 

 in the form of dust from one region to another, and almost every- 

 where the soil is pierced by worms, insects, and the roots of plants. 

 Many geologists have been disposed to refer the absence of stratifica- 

 tion in such formations to the sudden and tumultuous action of 

 floods, by which dense masses of mud were thrown down rapidly and 

 uninterruptedly ; but I believe that the absence of divisional planes 

 or marks of successive deposition has arisen, not from the want of 

 intermittent action, but because the amount of annual deposit has 

 been so slight, and because it has taken place on ground not 

 permanently submerged. There may be found in deposits of this class 

 examples of every gradation, from a stratified to an unstratified con- 

 dition. 



In European river-loams we occasionally observe isolated pebbles 

 and angular pieces of stone which have been floated by ice to the 

 places where they now occur ; but no such coarse materials are met 

 with in the plains of Egypt. Above and below the first cataract, an- 

 cient river terraces composed of fluviatile deposits have been observed 

 by Dr. Adams and others at various elevations above the present allu- 

 vial plain of the Nile. In these old river-formations — some of which 

 are 30, others 100, and others several hundred feet above the river — 



