26 PLEISTOCENE MAMMALIA. 



feeding grounds in Northern Europe and Asia, gradually passed southwards, until at last 

 it reached its maximum, at the time when Scandinavia, Great Britain, and Ireland, lay 

 buried under an enormous ice-sheet. The mixed character of the mammalia of the Lower 

 Brick-earths is just what might have been expected from any such migration as this. The 

 remains of the Pleistocene species, the Mammoth, Woolly Rhinoceros, and Cave Lion, are 

 quite as abundant as those of the Pleiocene E. hemifcechus, B. megarhinus, and JElephas 

 antiqutis, and prove that the former animals were in joint occupation of the region at the 

 time, and that the Pleiocene animals were still in competition with the new comers in 

 the district ; and that the latter should have been followed in the course of time by a stray 

 Musk Sheep is not at all to be wondered at. On this view, the Lower Brick-earths of the 

 Thames Valley may be ascribed, with tolerable certainty, to the age when the temperature 

 was gradually becoming lowered, towards the beginning of the Glacial period, rather than 

 to that during which vast herds of Reindeer lived on the site of London, and at Windsor, 

 while the gravels were being accumulated, which are proved by the foreign pebbles, 

 which they contain, to be posterior in date to the submergence of central and 

 northern Britain beneath the waves of the sea. During this later period the evidence 

 is conclusive, that the arctic division of the Pleistocene mammalia, — the Reindeer, 

 Gliltton, Musk Sheep, Marmot, and Spermophilus, had firm hold on the country, 

 and the Reindeer ranged over the whole of Great Britain, which was free from glaciers, 

 only comparable in number to the great migratory bands now living in northern 

 Siberia. Had the Lower Brick-earths of the Thames Valley been deposited at this time, 

 the Reindeer could hardly have failed to have been represented in the large collections of 

 mammaUa from Ilford, Crayford, Erith, and Grays Thurrock, since it is so abundant in 

 the river deposits higher up the valley of the Thames. They must therefore be earlier or 

 later in geological age ; and from the facts which I have brought forward, it seems to me 

 that they must be earlier, or before the maximum amount of cold was reached in the 

 Glacial period in Great Britain. 



This view of the high antiquity of the Lower Brick-earths in the Thames Valley, is 

 not held by the great authority on river deposits, Mr. Prestwich,^ who believes, because of 

 their slight elevation above the present level of the Thames, they must belong to a late 

 division of the Post-pleiocene, or Pleistocene age. There seem to me, however, to be 

 insuperable objections to the view that, in every case, the level will give the relative age of 

 the deposit. It is certain that, if all the superficial deposits in a given valley, say the 

 valley of the Thames, had been left by the ancient representatives of the present rivers, at 

 different levels above their present courses, those levels will give the relative antiquity of 

 the beds of sand or gravel in question, j^rovided that the land has remained stationary. 

 The extent to which the valley is cut down will give a rough sort of idea of the lapse of 

 past time. But if the land were elevated in one place, and depressed in another, as we are 



' Prestwich, ' Geol. Mag.,' vol. i, p. 245. 



