352 HISTORICAL PALEONTOLOGY. 



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 (Elephas priniigenius) 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 

 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- 



