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 Post-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 

 (Ldit.fltivms, 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 {Ekp/ias 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 



