340 HISTORICAL PALAAONTOLOGY. 
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 accompany- 
ing diagram will explain the origin and nature of these de- 
posits (fg. 255). When a river begins to occupy a particular 
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 rocks. (After Sir Charles Lyell.) 
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 charac- 
ter 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 
