154 
ELEMENTS OE GEOLOGY. 
position in the valley-plain, encroaching gradually on one 
bank, near which there is deep water, and deserting the oth¬ 
er or opposite side, where the channel is growing shallower, 
being destined eventually to be converted into land. Where 
the current runs strongest, coarse gravel is swept along, and 
where its velocity is slackened, first sand, and then only the 
finest mud, is thrown down. A thin film of this fine sed¬ 
iment is spread, during floods, over a wide area, on one, or 
sometimes on both sides, of the main stream, often reaching 
as tar 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 60 feet towards the central parts of the valley. 
Everywhere it consists of the same homogeneous mud, des¬ 
titute of stratification—the only signs of successive accumu¬ 
lation 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. 
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. 
In some parts of the valley of the Rhine the accumulation 
of similar loam, called in Germany “ loess,” has taken place 
on an enormous scale. Its color is yellowish-gray, and very 
homogeneous; and Professor Bischoff has ascertained, by 
analysis, that it agrees in composition with the mud of the 
Nile. Although for the most part unstratified, it betrays in 
some places marks pf stratification, especially where it con¬ 
tains calcareous concretions, or in its lower part where it 
rests on subjacent gravel and sand which alternate with 
each other near the junction. About a sixth part of the 
whole mass is composed of carbonate of lime, and there is 
usually an intermixture of fine quartzose and micaceous 
sand. 
Although this loam of the Rhine is unsolidified, it usually 
terminates where it has been undermined by running water 
in a vertical cliff, from the face of which shells of terrestrial, 
fresh-water and amphibious mollusks project in relief. These 
shells do not imply the permanent sojourn of a body of fresh 
water on the spot, for the most aquatic of them, the Siicci- 
