454 



NA TURE 



{Sept. 6, 1883 



markings for plants, even good botanists, when not familiar with 

 the chemical and mechanical condition- of fos-ilisation, and with 

 the present phenomena of tidal shores, are quite as easily misled, 

 1I1 ugh they are very prone, on the other hand, to regard land- 

 plants of some complexity, when badly pre erved, as mere algae. 

 In these circumstances it is very difficult to secure any consensus, 

 and the truth is only to be found by careful observation of 

 competent men. One trouble is that these usually obscure 

 markings have been despised by the greater number of paheonto- | 

 logists, and probably would not now be so much in controversy 

 were it not for the use made of them in illustrating supposed 

 genies of plants. 

 It would be wrong to close this address without some reference 1 

 to that which is the veritable pons asinorum of the science, the 

 great and much debated glacial period. I trust that you will 

 nit suppose that, in the end of an hour's address, I am about 

 to discuss this vexed question. Time would fail me even to 

 name the hosts of recent authors who have contended in this 

 arena. I can hope only to point out a few landmarks which may 

 aid the geological adventurer in traversing the slippery and 

 treacherous surface of the hypothetical ice-sheet of pleistocene 

 times, and in avoiding the yawning crevasses by which it is 

 traversed. 



No conclusions of geology seem more certain than that great 

 changes of climate have occurred in the course of geological 

 time ; and the evid nee of this in that comparatively modern 

 period which immediately preceded the human age is so striking 

 that it has come to be known as preeminently the ice age, while, 

 in the preceding tertiary periods, temperate conditions seem to 

 have prevailed even to the Pole. Of the many theories as to 

 these changes which have been proposed, two seem at present to 

 divide the suffrages of geologists, either alone, or combined with 

 each other. These ate, (1) the theory of the precession of the 

 equinoxes in connection with the varying eccentricity of the 

 earth's orbit, advocated more especially by Croll ; and (2) the 

 different distribution of land and water as affecting the reception 

 and radiation of heat and the ocean currents, — a theory ably 

 propounded by I yell, and subsequently extensively adopted, 

 either alone or with the previous one. One of the<c view s may 

 be called the astronomical ; the other, the geographical. I confess 

 that I am inclined to accept the second or Lyellian theory for 

 such reasons as the following : I. Great elevations and depressions 

 of land have occurred in and since the pleistocene, while the 

 alleged astronomical changes are not certain, more especially in 

 regard to their probable effect on the earth ; 2. When the rival 

 theories are tested by the present phenomena of the Southern 

 Polar region and the North Atlantic, there seem to be geo- 

 graphical causes adequate to account for all except extreme and 

 unproved glacial conditions ; 3. The a-tronomical cause would 

 suppo-e regularly recurring glacial periods of which there is no 

 evidence, and it would give to the latest glacial age an antiquity 

 which seems at variance with all other facts ; 4. In those more 

 northern regions where glacial phenomena are most pronounced, 

 the theory of floating sheets of ice, with local glaciers descending 

 to the sea, seems to meet all the conditions of the case; and 

 these would be obtained, in the North Atlantic'at least, by very 

 moderate changes of level, causing, for example, the equatorial 

 current to flow into the Pacific, instead of running northward as 

 a gulf stream ; 5. The geographical theory allows the supposition 

 not merely of vicissitudes of climate quickly following each 

 other in unison with the movements of the surface, but allows 

 also of that near local approximation of regions wh< Uy covered 

 with ice and snow, and others comparatively temperate, which 

 we see at present in the north. 



If, however, we are to adopt the geographical theory, we must 

 avoid extreme views ; and this leads to the inquiry as to the 

 evidence to be found for any such universal ami extreme 

 glaciation as is demanded by some geologists. 



The only large continental area in the northern hemisphere 

 supposed to be entirely ice and snow clad is Greenland ; and 

 this, so far as it goes, is certainly a local case, for the ice and 

 snow nf Greenland extend to the south as far as 60° N. latitude, 

 while both in Norway and in the interior of North America the 

 climate in that latitude permits the growth of cereals. Further, 

 Grinnel Land, which is separated from North Greenland only 

 by a narrow sound, has a comparatively mild climate, and, as 

 Nares has shown, is covered with verdure in summer. Still 

 further, Nordenskjbld, one of the most experienced Arctic ex- 

 plorers, hold- that it is probable that the interior of Greenland 

 'If verdant in summer, and is at this moment preparing to 



attempt to reach this interior oasis. Nor is it difficult, with the 

 aid of the facts cited by Woeikoff and Whitney, 1 to perceive 

 the cause of the exceptional condition of Greenland. To give 

 ice and snow in large quantities, two conditions are required, — 

 first, atmospheric humidity ; and, secondly, cold precipitating 

 regions. Both of these conditions meet in Greenland. Its high 

 coast-ranges receive and condense the humidity from the sea on 

 both sides of it and to the south. Hence the vast accumulation 

 of its coast snow-fields, and the intense discharge of the glaciers 

 emptying out of its valleys. When extreme glacialists point to 

 Greenland, and ask us to believe that in the glacial age the whole 

 continent of North America as far sou'h as the latitude of 40° 

 was covered with a continental glacier, in some places several 

 thousands of feet thick, we may well ask, first, what evidence 

 there is that Greenland, or even the Antarctic continent, at 

 preseut shows such a condition ; and, secondly, whether there 

 ex^ts a possibility that the interior of a great continent could 

 ever receive so large an amount of precipitation as that required. 

 So far as present know ledge exists, it is certain that the meteor- 

 ologist and the physicist mu.t answer both questions in the 

 negative. In short, perpetual glaciers must be local, 



and cannot be continental, because of the vast amount of evapo- 

 ration and conden ation required. These can only be possible 

 where comparatively warm seas supply moisture to cold and 

 elevated land ; and this supply cannot, in the nature of things, 

 penetrate far inland. The actual condition of interior Asia and 

 interior America in the higher northern latitudes affords positive 

 proof of this. In a state of partial submergence of our northern 

 continents, we can readily imagine glaciation by the combined 

 action of local glaciers and great ice-floes ; but, in whatever way 

 the phenomena of the boulder clay and of the s i-called terminal 

 moraines are to be accounted for, the theory of a continuous 

 continental glacier must be given up. 



I cannot better indicate the general bearing of facts, as they 

 present themselves to my mind in connection with this subject, 

 than by referring to a paper by Dr G. M. Dawson on the distri- 

 bution of drift nver the great Canadian plains east of the Rocky 

 Mountains.- I am the more inclined to refer to this, because of 

 its recency, and because I have so often repeated similar con- 

 clusions as to eastern Canada and the region of the Great 

 Lakes. 



The great interior plain of western Canada, between the 

 Laurentian axis on the east and the Rocky Mountains on the 

 west, is seven hundred miles in breadth, and is covered with 

 glacial drift, presenting one of the greatest examples of this 

 deposit in the world. Proceeding eastward from the base of the 

 Rocky Mountains, the surface, at firs; more than four thousand 

 feet above the sea-level, descends by successive steps to twenty- 

 five hundred feet, and is based on cretaceous and 1 .aramie rocks, 

 covered by boulder clay and sand, in some places from one 

 hundred to two hundred feel in depth, and filling up preexisting 

 hollows, though itself sometimes piled into ridges. Near the 

 Rocky Mountain, the bottom of the drift consists of gravel not 

 glaciated. This extends to about one hundred miles ea-t of the 

 mountains, and mu-t have been swept by water out of their 

 valleys. The boulder clay resting on this deposit is largely 

 made up of local debris, in so far as it- paste is concerned. It 

 contains many glaciated boulders and stones from the Laurentian 

 region to the east, and also smaller pebbles from the Rocky 

 Mountains ; so that at the time of its formation there must have 

 been driftage of large stones for seven hundred nides or more 

 from the east, and of smaller stones from a less distance on the 

 west. The former kind of material extends to the base of the 

 mountains, and to a height of more than four thousand feet. 

 One boulder is mentioned as bein,' forty-two by forty by twenty 

 feet in dimensions. The highest Laurentian boulders seen were 

 at an elevation of forty-six hundred and sixty f ie base 



of the Rocky Mountains. The boulder clay, when thick, can 

 be seen to be rudely stratified, and at one place includes beds of 

 laminated clay with compre-sed peat, similar to the forest beds 

 described by Worthen and Andrews in Illinois, and the so-called 

 inter-glacial beds described by Hmde on Lake Ontario. The 

 leaf-beds on the Ottawa River, and the drift-trunks found in the 

 boulder clay of Manitoba, belong to the same category, and 

 indicate that throughout the glacial period there were many forest 

 oases far to the north. In the valleys of the Rocky Mountains 

 opening on these plains there are evidences of large local glaciers 



' "Memoir on Glaciers," Geol. Soc. Berlin. 1B81. " Climat : c Changes," 

 Boston, 1883. " Scitnce, July 1, 1883. 



