240 THE GLACIAL OR ICE EPOCH. 



clays of great thickness, erratic Hocks of enormous size, 

 polished, striated, and grooved rocks, moraine-like mounds 

 of gravel, and smoothed and rounded surfaces are so com- 

 mon in Britain, Northern Europe, and North America, 

 that geologists are driven to the conclusion of a glacial 

 epoch; a long period intermediate between the Tertiary 

 and the Current era, when all the northern hemisphere, 

 down to the 40th or 42d parallel, was under the influences 

 of an icy climate like that which now prevails within the 

 arctic and antarctic circles. 



Geologists have long been at variance, and in some in- 

 stances are still at variance, as to whether the phenomena 

 are to be attributed more to land-ice or to sea-ice, to glaciers 

 or to icebergs. As we became better acquainted, however, 

 with the operations of ice in such regions as the Alps, 

 Himalayas, Scandinavia, Spitzbergen, Greenland, and the 

 polar seas, such differences of opinion grew less, and com- 

 petent authorities seem to be agreed that we must call in 

 both agencies, and this during the successive stages of sub- 

 sidence and re-elevation which we have already endeavoured 

 to describe. Indeed, it is impossible to conceive of a glacial 

 climate over any large portion of the earth's surface without 

 seeing that it must affect sea and land alike j and that if 

 there is any brief period of thaw, like the existing arctic 

 summer, the ice must be set in motion both on land and 

 water.* Once set in motion, each would contribute its 

 quota to the general result the land-ice grinding and 

 smoothing and rounding the rocky surface in its descent to 

 the sea, and the sea-ice ploughing the shallower sea-bed as 

 it floated away to drop its burden of boulders and debris 



* Even without any great degree of summer thaw, a mobile mass like 

 ice and snow would be urged forward by its own accumulating weight, 

 and this over heights and hollows, so long as the head pressure remained 

 sufficiently powerful. See preceding Sketch, p. 221. 



