72 THE POPULAR SCIENCE MONTHLY. 



als which glaciers would have swept away, sometimes contains marine 

 shells, or passes into marine clays in its horizontal extension, and in- 

 variably in its imbedded 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 there 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 gener- 

 ally 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 extension 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. Recently 

 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 gen- 

 eral, which have occurred since the close of the glacial period, are 

 scarcely appreciable ; little erosion of river-valleys or of coast-terraces 

 has occurred. The calculated recession of water-falls and of produc- 

 tion of lake-ridges lead to the same conclusion. So do the recent state 

 of bones and shells in the pleistocene deposits and the perfectly mod- 

 ern facies of their fossils. On such evidence the cessation of the gla- 

 cial 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 un- 

 certain in geology. This 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 can not be discussed 

 in the closing sentences of an hour's address. I shall only draw from 

 them one practical inference. Since the comparatively short post-gla- 

 cial and recent periods apparently include the whole of human history, 

 we are but new-comers on the earth, and therefore 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 century 

 ago. We have reason for surprise, in these circumstances, that it has 



