496 Proceedings of the Royal Irish Academy. 



crags, the latter are synchronous with the lower Continental boulder- 

 clay, in -vrhich these same species are first met with. This reasoning 

 might be found fault with, but I have clearly shown (p. 462) that the 

 Tast immigration into Europe of Siberian ITammals took place after 

 the deposition of the lower Continental boulder-clay, duiing the so- 

 called Interglacial phase of the Glacial Period. Now, as the advance 

 guard of this migration reached England during the time when the 

 Eorest-bed was laid down, the supposition of the contemporaneousness 

 of the newer crags with the lower Continental boulder-clay seems to 

 me correct. There is no reason to suppose that the interglacial era or 

 Forest-bed Period was characterised by a much milder climate than that 

 preceding it, but, as it was probably much drier, the glaciers which 

 had formed on the Alps and in Scandinavia, receded considerably. 

 The immediate result was a diminution in the amoimt of detritus 

 carried to the Xorth European Sea by icebergs, so that more extensive 

 colonies of marine animals were able to establisli themselves on the 

 sea floor than during the preceding stage of the Glacial Period. The 

 narrow straits, which had formed across the Tchtrnosjem district ia 

 Central Russia, between the Northern Sea and the greatly enlarged 

 Caspian, ceased to exist in the Interglacial phase, owing to the 

 gradual withdrawal of the Arctic waters from Northern Europe. A 

 sliglit refrigeration of the Siberian climate was the consequence, and 

 the barrier which prevented egress to Europe being now removed, the 

 Northern Asiatic fauna swarmed across the plains of the newly-opened 

 continent. 



During all this time, Scandinavia remained connected in the north 

 with Greenland, and in the south and west with Scotland and Ireland, 

 but it had no direct communication with the Continent, being separated 

 from it by the North European Sea. England and France were united 

 throughout the Glacial Period, but the connexion between the former 

 and Ireland broke down during, or shortly after, the deposition of the 

 Forest-bed, so that none of the Siberian migrants, which now poured 

 into England from the Continent, reached Ireland. 



Extensive areas of Eastern Yorkshire, according to Prof. James 

 Geikie (35 a, p. 3o4), are covered by boulder-clay, with intermediate 

 beds of gravel and sand. Two divisions, viz. a lower and upper clay, 

 are recognizable. The former contains many far-travelled erratics 

 and marine shells, and only a moderate amount of local chalk. But 

 in the purple or upper clay, erratics of home-origin are more con- 

 spicuous (p. 364). Prof. Geikie explains these facts in the following 

 manner : — That the basement clay is the direct product of a great 



