SIGNS OF PAST GLACIATION. 53 



by various agencies ; as, for example, by the forces which 

 fracture the earth's crust, and shove one portion past 

 another, producing what is called a slichen-side. Or, 

 again, avalanches or land-slides might be competent to 

 produce the results over limited and peculiarly situated 

 areas. Icebergs, also, and shore ice which is moved back- 

 wards and forwards by the waves, would produce a cer- 

 tain amount of such grooving and scratching. But the 

 phenomena to which we refer are so extensive, and occur 

 in such a variety of situations, that the movement of 

 glacial ice is alone sufficient to afford a satisfactory ex- 

 planation. Moreover, in Alaska, Greenland, Norway, 

 and Switzerland, and wherever else there are living 

 glaciers, it is possible to follow up these grooved and 

 striated surfaces till they disappear underneath the ex- 

 isting glaciers which are now producing the phenom- 

 ena. Thus by its tracks we can, as it were, follow 

 this monster to its lair with as great certainty as we 

 could any animal with whose footprints we had become 

 familiar. 



2. The till, or boulder-clay. 



A second sign of the former existence of glaciers over 

 any area consists of an unstratified deposit of earthy 

 material, of greater or less depth, in which scratched 

 pebbles and fragments of rock occur without any definite 

 arrangement. 



Moving water is a most perfect sieve. During floods, 

 a river shoves along over its bed gravel and pebbles of 

 considerable size, whereas in time of low water the cur- 

 rent may be so gentle as to transport nothing but fine 

 sand, and the clay will be carried still farther onwards, to 

 settle in the still water and form a delta about the river's 

 mouth. The transporting capacity of running water is 

 in direct ratio to the sixth power of its velocity. Other 

 things being equal, if the velocity be doubled, the size of 

 the grains of sand or gravel which it transports is in- 



