endered more clearly intelligible.

Every river produces at the present day beds of fine mud and
loam, and accumulations of gravel, which it deposits at various
parts of its course--the gravel generally occupying the lowest
position, and the finer sands and mud coming above. Numerous
deposits of a similar nature are found in most countries in various
localities, and at various heights above the present channels of
our rivers. Many of these fluviatile (Lat. _fluvius_, a river)
deposits consist of fine loam, worked for brick-making, and known
as "Brick-earths;" and they have yielded the remains of numerous
extinct Mammals, of which the Mammoth (_Elephas primigenius_) is
the most abundant. In the valley of the Rhine these fluviatile
loams (known as "Loess") attain a thickness of several hundred
feet, and contain land and fresh-water shells of existing species.
With these occur the remains of Mammals, such as the Mammoth and
Woolly Rhinoceros. Many of these Brick-earths are undoubtedly
Post-Glacial, but others seem to be clearly "inter-glacial;" and
instances have recently been brought forward in which deposits
of Brick-earth containing bones and shells of fresh-water Molluscs
have been found to be overlaid by regular unstratified boulder-clay.

The so-called "Valley-gravels," like the Brick-earths, are fluviatile
deposits, but are of a coarser nature, consisting of sands and
gravels. Every river gives origin to deposits of this kind at
different points along the course of its valley; and it is not
uncommon to find that there exist in the valley of a single river
two or more sets of these gravel-beds, formed by the river itself,
but formed at times when the river ran at different levels, and
therefore formed at different periods. These different accumulations
are known as the "high-level" and "low-level" gravels; and a
reference to the accompanying diagram will explain the origin
and nature of these deposits (fig. 255). When a river begins
to occupy a particular line of drainage, and to form its own
channel, it will deposit fluviatile sands and gravels along its
sides. As it goes on deepening the bed or valley through which
it flows, it will deposit other fluviatile strata at a lower
level beside its new bed. In this way have arisen the terms
"high-level" and "low-level" gravels. We find, for instance, a
modern river flowing through a valley which it has to a great
extent or entirely formed itself; by the side of its imm