Overlying the same Baltic deposits comes the lower boulder-clay, reaching a thickness of several hundred feet in some parts of Germany. It presents, like the upper clay, frequent interstratification with well-bedded deposits of sand and gravel. The scarcity of marine mollusca, the occurrence of striated surfaces, and the occasional presence of so-called giants' kettles, appear to favour the view, which at present is generally adopted by both British and Continental geologists, that the boulder-clay owes its origin to land-ice. I have stated on several occasions that the view of the marine origin of the boulder-clay agrees best with the known facts of distribution, and with the history of the European fauna (pp. 80-86, and p. 129). It may be urged that if the lower boulder-clay were contemporaneous with the British Crags which succeeded the Red Crag, how can we explain the fact that these crags contain plenty of shells, while in the lower continental boulder-clay there are scarcely any?
But as yet our knowledge of the conditions of life of the marine mollusca and of their distribution is extremely scanty. We are apt to imagine that the bottom of the sea is covered by a more or less uniform thick layer of shells; but whenever a careful survey of the nature of the deposits now forming there has been made, such is by no means found to be the case. Some of the best results obtained by that useful body, the Liverpool Marine Biological Committee, have been precisely in this direction. A most interesting account has been published by Professor Herdman and Mr. Lomas on the floor deposits of the Irish Sea, in which the authors state (p. 217), that "a place may be swarming with life and yet leave no trace of anything capable of being preserved in the fossil state, whereas in other places, barren of living things, banks of drifted and dead shells may be found, and remain as a permanent deposit on the ocean floor."
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.