166 SUBMARINE GLACIAL DRIFT. [Ch. XR 



year after year of large masses of coast-ice and occasional icebergs, 

 might be sunk to a depth, of several hundred fathoms. By the con- 

 stant depression of land, the coast would recede farther and farther 

 from the successively formed zones of polished and striated rock, each 

 outer zone becoming in its turn so deep under water, as to be no 

 longer grated upon by the heaviest icebergs. Such sunken areas 

 would then simply serve as receptacles of mud, sand, and boulders 

 dropped from melting ice, perhaps to a depth scarcely, if at all, in- 

 habited by testacea and zoophytes. Meanwhile, during the. forma- 

 tion of the unstratified and unfossiliferous mass in deep water, the 

 smoothing and furrowing of shoals and beaches would still go on else- 

 where upon and near the coast in full activity. If at length the sub- 

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

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

 reconverted 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 denuded, would 

 have its materials rearranged 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 ; sec- 

 ondly, an unstratified and unfossiliferous mass called till, for the most 

 part of much older date than the preceding, with angular erratics, or 

 with boulders interspersed ; and thirdly, beneath the whole, a surface 

 of polished and furrowed rock. 



If we reflect on the vast area over which the dispersion of marine 

 glacial drift is now in progress, we shall at once see that it must 

 equal, if it does not greatly exceed, the space over which glaciers and 

 continental ice are moving. It would, therefore, have been in the 

 highest degree perplexing if we had not met with proofs of subma- 

 rine glaciation on a most extensive scale, including all the phenomena 

 of polishing, scratching, furrowing, and rounding of rocky surfaces, 

 and the transportation of erratics and finer materials ; seeing that 

 there is so much evidence everywhere in Europe and North America 

 of the conversion of sea into land, as well as land into sea, since the 

 commencement of the glacial epoch. 



But although a large portion of the drift of North America has 

 been due, like that of Europe, to floating ice and a period of sub- 

 mergence, that continent has also had its land-ice, and its centres of 

 dispersion of erratic blocks. The White Mountains of New Hamp- 

 shire, lat. 44° N., the loftiest of which is more than 6000 feet high, 

 may be cited as an example ; and the late Professor Hitchcock in- 

 ferred that some of the highest hills in Massachusetts once sent down 

 their glaciers into the lower country. I have already mentioned that 

 in Europe several quadrupeds of living, as well as extinct, species 

 were common to pre-glacial and post-glacial times. In like manner 

 there is reason to suppose that in North America much of the ancient 



