Owing to the fact of the peculiar geographical position of Scandinavia at this time--an isthmus of land with a high mountain range lying between the warm Atlantic and the cold Arctic Sea--the snowfall must have been excessive, and large glaciers were evidently forming. These produced icebergs as soon as the lower parts had advanced to the Baltic coast-land and deposited their detritus in the sea. Immense masses of mud and stones were thus cast to the bottom of the sea, and under these circumstances no delicate mollusca or other marine life probably could have developed within a considerable distance from the shore. To judge from the direction pursued by the majority of the boulders from their source of origin, the prevailing current during the deposition of the lower boulder-clay was from north-west to south-east. It is possible that little marine life, except free-swimming forms, would have been able to live within the Russian area of this sea. But the free-swimming larvæ of molluscs and other surface species were not prevented from passing from the White Sea south-westward, and in sheltered localities where little or no mud deposition was going on, these no doubt might have developed into adults on the sea-floor. It is quite conceivable, therefore, that in one portion of the North European Sea, which was fully exposed to the destructive influences of the iceberg action, the fauna was scanty or totally absent, while in another part there lived a fairly abundant one. The unfossiliferous state of the lower continental boulder-clay does not, therefore, offer any serious difficulty to the supposition that some of the so-called Newer Pliocene Crags of the east coast of England were laid down at the same time by the same sea. This would also explain how the Arctic species come to inhabit the Caspian, as the old Aralo-Caspian Sea could have had some communication (Fig. 12, p. 156) with the North European Sea. And this again offers an explanation of the otherwise mysterious occurrence of the Caspian _Dreyssensia polymorpha_ in the lower continental boulder-clay.