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 immediate channel we may find gravels, sand, and loam (fig. 255, 2 2’) deposited by the river flowing in its present bed.  These are recent fluviatile or alluvial deposits.  At some distance from the present bed of the river, and at a higher level, we may find other sands and gravels, quite like the recent ones in character and origin, but formed at a time when the stream flowed at a higher level, and before it had excavated its valley to its present depth.  These (fig. 255, 3 3’) are the so-called “low-level gravels” of a river.  At a still higher level, and still farther removed from the present bed of the river, we may find another terrace, composed of just the same materials as the lower one, but formed at a still earlier period, when the excavation of the valley had proceeded to a much less extent.  These (fig. 255, 4 4’) are the so-called “high-level gravels” of a river, and there may be one or more terraces of these.

[Illustration:  Fig. 255.—­Recent and Post-Pliocene Alluvial Deposits. 1, Peat of the recent period; 2, Gravel of the modern river:  2’, Loam of the modern river; 3.  Lower-level valley-gravel with bones of extinct Mammals (Post-Pliocene); 3’, Loam of the same age as 3; 4.  Higher-level valley-gravel (Post-Pliocene); 4’, Loam of the same age as 4; 5.  Upland gravels of various kinds (often glacial drift); 6, Older rock. (After Sir Charles Lyell.)]