THE QUATERNARY PERIOD. 339 
conditions of climate were once more re-established, various 
deposits were formed in the northern hemisphere, which are 
found to contain the remains of extinct Mammals, and which, 
therefore, are clearly of Post-Pliocene age. To these deposits 
the general name of ost-Glacial formations is given ; but it is 
obvious that, from the nature of the case, and with our present 
limited knowledge, we cannot draw a rigid line of demarcation 
between the deposits formed towards the close of the Glacial 
period, or during warm “ interglacial” periods, and those laid 
down after the ice had fairly disappeared. Indeed it is ex- 
tremely improbable that any such rigid line of demarcation 
should ever have existed ; and it is far more likely that the 
Glacial and Post-Glacial periods, and their corresponding de- 
posits, shade into one another by an imperceptible gradation. 
Accepting this reservation, we may group together, under the 
general head of “ Post-Glacial Deposits,” most of the so-called 
‘‘ Valley - gravels,” “ Brick - earths,” and ‘“‘ Cave - deposits,” to- 
gether with some ‘raised beaches” and various deposits of 
peat. ‘Though not strictly within the compass of this work, 
a few words may be said here as to the origin and mode 
of formation of the Brick-earths, Valley-gravels, and Cave- 
deposits, as the subject will thus be rendered 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 (Zdephas 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 con- 
tain 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 regu- 
lar unstratified boulder-clay. 
The so-called ‘ Valley-gravels,” like the Brick-earths, are 
