xiv SURFACE CEOL< j 



they could not have supplied the block in question 

 cxcei vention of tome unusual agent which 



moved it resent position, he will establish that 



at all events the boulder is a transported one. 



What may have been the agent of transport must be 

 .ir evidence in each case. In a 

 >cks within reach of the present or former 

 currents may have been moved downward by river 

 floods. Waves can throw up blocks of considerable 

 site, and even quarry them out of their solid U-ds in the 

 parent rock at heights of seventy feet or more above high 

 water. To the freezing of the water of rivers, lakes, or 

 the sea around shore-blocks, and the subsequent breaking 

 up of the ice, the transport of considerable masses of 

 rock along shore has been due. Icebergs have been 

 observed at sea with blocks of stone and heaps of rock- 

 rubbish lying upon them. Glaciers transport enormous 

 quantities of loose blocks of rock and earth from the 

 upper valleys. Undoubtedly ice has been the great agent 

 in the distribution of erratic blocks : but whether in the 

 form of floating ice, of glaciers, or of a great general sheet 

 of land-ice, must be decided in each district, not only 

 from the evidence of the blocks themselves, but from t he- 

 other data obtainable as to the glaciation of the country. 



Glaciatio*. Only a very few words are possible here 

 on this wide and fascinating subject, regarding which of 

 course the reader has ample means of information from 

 the voluminous literature already devoted to it Certainly 

 among the many superficial characters of interest found 

 in the northern parts of the northern hemisphere, as well 

 as in mountain tracts in other regions of the globe, few 



