TO J. Croll — Evidence of former Glacial Periods. 



pect to find in temperate regions erratic blocks, imbedded here 

 and there in the stratified rocks, which may have been trans- 

 ported by icebergs and dropped into the sea. But unless the 

 glaciers of such epochs reached the sea, we could not possibly 

 possess even this evidence. This sort of evidence, when found 

 in low latitudes, ought to be received as evidence of the exist- 

 ence of former glacial epochs ; and, no doubt, would have 

 been so received had it not been for the erroneous idea that, 

 if these blocks had been transported by ice, there ought in ad- 

 dition to have been found striated stones, bowlder clay, and 

 other indications of the agency of land-ice. 



It is, of course, by no means the case that all erratics are 

 transported by masses of ice broken from the terminal front 

 of glaciers. The "ice foot," formed by the freezing of the 

 sea along the coasts of the higher latitudes, carries seawards 

 quantities of blocks and debris. Again, stones and bowlders 

 are frequently frozen into river ice, and when the ice breaks 

 up in spring are swept out to sea, and may be carried some 

 little distance before they are dropped. But both these cases 

 can occur only in regions where the winters are excessive / nor 

 is it at all likely that such ice-rafts will succeed in making a 

 long voyage. If, therefore, the erratics occasionally met with 

 in certain old geological formations in low latitudes were 

 really transported from the land by an ice-foot or a raft of 

 river-ice, we should be forced to conclude that very severe 

 climatic conditions must have obtained in such latitudes at the 

 time the erratics were dispersed. 



Why we now have, comparatively speaking, so little direct 

 evidence of the existence of former glacial periods will be 

 more forcibly impressed upon the mind if we reflect on how 

 difficult it would be, in a million or so of years hence, to find 

 any trace of what we now call the glacial epoch. The striated 

 stones would by that time be all, or nearly all, disintegrated, and 

 the till washed away and deposited in the bottom of the sea as 

 stratified sands and clays. And when these became consoli- 

 dated into rock and were raised into dry land, the only evi- 

 dence that we should probably then have that there ever had 

 been a glacial epoch would be the presence of an occasional 

 large block of the older rocks found imbedded in the upraised 

 formation. We could only infer that there had been ice at 

 work from the fact that by no other known agency could we 

 conceive such a block to have been transported and dropped 

 in a still sea. 



Few geologists probably believe that during the Middle 

 Eocene and the Upper Miocene periods onr country passed 

 through a condition of glaciation as severe as it has done dur- 

 ing the Post-pliocene period ; yet when we examine the sub- 



