200 



SCIENCE. 



[Vol.. ir., No. 28. 



cable in tliose western regions where they attain 

 their greatest development. It is plain that in the 

 north it marks the western limit of the deep water of 

 a glacial sea, which at some periods extended much 

 fartljer west, perhaps with a greater proportionate de- 

 pression in going westward, and on which heavy ice 

 from the Lauren tian districts on the east was wafted 

 south-westward by the arctic currents, while lighter 

 ice from the Eocky Mountains was being borne east- 

 ward from these mountains by the prevailing wester- 

 ly winds. We thus have in the west, on a very wide 

 scale, the same phenomena of varying submergence, 

 cold currents, great ice-floes, and local glaciers pro- 

 ducing icebergs, to which I have attributed the 

 bowlder clay and upper bowlder drift of eastern 

 Canada. 



A few siihsidiary points I may be pardoned for 

 mentioning hei'e. The rival theories of the glacial 

 period are often characterized as those of landglacia- 

 tion and sea-borne icebergs. But it must be remem- 

 bered, that those who reject the idea of a continental 

 glacier hold to the existence of local glaciers on the 

 high l.-xnds more or less extensive during different 

 portions of the great pleistocene submergence. 

 They also believe in the extension of these glaciers 

 seawards and partly water-borne, in the manner so 

 well explained by Mattieu Williams; in tlK existence 

 of those vast floes and fields of current- and tide-borne 

 ice whose powers of transport and erosion we now 

 know to be so great; and in a great submergence 

 and re-elevation of the land, bringing all parts of it 

 and all elevations up to five thousand feet succes- 

 sively under the influence of these various agencies, 

 along with those of the ocean-currents. They also 

 hold, that, at the beginning of the glacial submer- 

 gence, the land was deeply covered by decomposed 

 rock, similar to that which still exists on the hills of 

 the southern states, and which, as Dr. Hunt has 

 shown, would afford not only earthy debris, but large 

 quantities of bowlders ready for transportation by 

 ice. 



I would also remark, that there has been the great- 

 est possible exaggeration as to the erosive action of 

 land-ice. In 1S0.5, after a visit to the alpine glaciers, 

 I maintained that in these mountains glaciers are 

 relatively protective rather than erosive agencies, and 

 that the iletritus which the glacier streams deliver 

 is derived mostly from the atmospherically wasted 

 peaks and cliffs that project above them. Since that 

 time many other observers have maintained like 

 views, and very recently Mr. Davis of Cambridge 

 and Mr. A. Irving have ably treated this subject. i 

 Smoothing and slriation of rocks are undoubtedly 

 important effects, both of land-glaciers and heavy sea- 

 borne ice; but the levelling and filling agency of 

 these is much greater than the erosive. As a mat- 

 ter of fact, as Newberry, Hunt, Belt, Spencer, and 

 others have shown, the glacial age has dammed up 

 vast numbers of old channels which it has been left 

 for modern streams partially to excavate. 



The till, or bowlder clay, has been called a 'ground 



' Proc. Boat, soc. nai. hiat., xxii. Journ. geoL soc. Lond., 

 Feb., 1SS3. 



moraine.' but there are really no alpine moraines at 

 all corresponding to it. On the other hand, it is 

 more or less stratified, often rests on soft materials 

 which glaciers would have swept away, sometimes 

 contains marine shells, or passes into marine clays 

 in its horizontal extension, and invariably in its em- 

 bedded bowlders and its paste shows an unoxidized 

 condition, which could not have existed if it had 

 been a sub-aerial deposit. When the Canadian till is 

 excavated, and exposed to the air, it assumes a brown 

 color, owing to oxidation of its iron ; and many of its 

 stones and bowlders break up and disintegrate under 

 the action of air and frost. These are unequivocal 

 signs of a sub-aqueous deposit. Here and tliere we 

 find associated with it, and especially near the bottom 

 and at the top, indications of powerful water-action, 

 as if of land-torrents acting at particular elevations 

 of the land, or heavy surf and ice action on coasts; 

 and the attempts to explain these by glacial streams 

 have been far from successful. A singular objection 

 sometimes raised against the sub-aqueous origin of 

 the till is its general want of marine remains, but 

 this is by no means universal; and it is well known 

 that coarse >conglomerates of all ages are generally 

 destitute of fossils, except in their pebbles; and it is 

 further to be observed, that the conditions of an ice- 

 laden sea are not those most favorable for the exten- 

 sion of marine life, and that the period of time 

 covered by the glacial age must have been short, 

 compared with that represented by some of the older 

 formations. 



This last consideration suggests a question which 

 might afford scope for another address of an hour's 

 duration, — the question how long time has elapsed 

 since the close of the glacial period. Kecently the 

 opinion has been gaining ground that the close of the 

 Ice age is very recent. Such reasons as the following 

 lead to this conclusion: the amount of atmospheric 

 decay of rocks and of denudation in general, which 

 have occurred since the close of the gl.acial period, 

 are scarcely appreciable; little erosion of river-val- 

 leys or of coast-terraces has occurred. The calcu- 

 lated recession of waterfalls and of production of 

 lake-ridges lead to the same conclusion. So do the 

 recent state of bones and shells in the pleistocene 

 deposits, and the perfectly modern facies of their 

 fossils. On such evidence the cessation of the glacial 

 cold and settlement of our continents at their present 

 levels are events which may have occurred not more 

 than six thousand or seven thousand years ago, 

 though such time estimates are proverbially uncer- 

 tain in geology. Tliis subject also carries with it 

 the greatest of all geological problems, next to that 

 of the origin of life; namely, the origin and early 

 history of man. Such questions cannot be discussed 

 in the closing sentences of an hour's addiess. I 

 shall only draw from them one practical inference. 

 Since the comparatively short post-glacial and recent 

 periods apparently include the whole of human his- 

 tory, we are but new-comers on the earth, and there- 

 fore have had little opportunity to solve the great 

 problems which it presents to us. But this is not all. 

 Geology as a science scarcely dates from a ccntuiy ago. 



