142 STRIATED PEBBLES AND BOULDERS. [Ch. XII. 



to a depth scarcely, if at all inhabited by testacea and zoophytes. Mean- 

 while, during the formation of the unstratified and unfossiliferous mass in 

 deeper water, the smoothing and furrowing of shoals and beaches would 

 still go on elsewhere upon and near the coast in full activity. If at length 

 the subsidence should cease, and the direction of the movement of the earth's 

 crust be reversed, the sunken area covered with drift would be slowly re- 

 converted into land. The boulder deposit, before emerging, would then for 

 a time be brought within the action of the waves, tides, and currents, so that 

 its upper portion, being partially disturbed, would have its materials re- 

 arranged and stratified. Streams also flowing from the land would in 

 some places throw down layers of sediment upon the till. In that case, 

 the order of superposition will be, first and uppermost, sand, loam, and 

 gravel occasionally fossiliferous ; secondly, an unstratified and unfossilifer- 

 ous mass, called till, for the most part of much older date than the pre- 

 ceding, with angular erratics, or with boulders interspersed ; and, thirdly, 

 beneath the whole, a surface of polished and furrowed rock. Such a 

 succession of events seems to have prevailed very widely on both sides 

 of the Atlantic, the travelled blocks having been carried in general from 

 the North Pole southwards, but mountain chains having in some cases 

 served as independent centres of dispersion, of which the Alps present 

 the most conspicuous example. 



It is by no means rare to meet with boulders imbedded in drift which 

 are worn flat on one or more of their sides, the surface being at the same 

 time polished, furrowed, and striated. They may have been so shaped 

 in a glacier before they reached the sea, or when they were fixed in the 

 bottom of an iceberg as it ran aground. We learn from Mr. Charles 

 Martins that the glaciers of Spitsbergen project from the coast into a sea 

 between 100 and 400 feet deep ; and that numbers of striated pebbles 

 or blocks are there seen to disengage themselves from the overhanging 

 masses of ice as they melt, so as to fall at once into deep water.* 



That they should retain such markings when again upraised above the 

 sea ought not to surprise us, when we remember that rippled sands, and 

 the cracks in clay dried between high and low water, and the foot-tracks 

 of animals and rain-drops impressed on mud, and other superficial 

 markings, are all found fossil in rocks of various ages. 



On the other hand, it is not difficult to account for the absence in 

 many districts of striated and scored pebbles and boulders in glacial 

 deposits, for they may have been exposed to the action of the waves on 

 a coast while it was sinking beneath or rising above the sea. No shingle 

 on an ordinary sea-beach exhibits such striae, and at a very short distance 

 from the termination of a glacier every stone in the bed of the torrent 

 which gushes out from the melting ice is found to have lost its glacial 

 markings by being rolled for a distance even of a few hundred yards. 



The usual dearth of fossil shells in glacial clays well fitted to preserve 

 organic remains may, perhaps, be owing, as already hinted, to the 



* Bulletin Soc GeoL de France, torn. iv. 2de ser. p. 1121. 



