PHYSICAL AND CLLMATAL CONDITIONS. m 



present time, and may continue during a period of eleva- 

 tion, until large portions of the drift are raised above the 

 sea-level and beyonil the inHuence of the waves, which 

 will attack only its sea front. J^ut the agent wliich gives 

 rise to this lietcrogeneous mass is pan ice, and the forma- 

 tion of boulder clay is very proljably a part of its work 

 over a vast area on the Lal)rudor coast at the present day, 

 throughout the labvrinth of islands which friui^e that 

 coast to a depth of 20 miles seawards." 



There can be no doubt, as Dr. G. M. IJawson has pointed 

 out in the case of the western plains and of l>ritish 

 Columl)ia, that the typical boulder-clay spread over the 

 country and filling pre-existing hollows is nuich more of the 

 luiture of the deposits now forming by field, tloe, and ])an 

 ice under water, than of anything of the nature of the 

 bottom moraine of a glacier. Its nuiterial may, however, 

 have been added to and its arrangement afi'ected by the bergs 

 thrown off from the foot of glaciers terminating in the sea. 



An important question arises here as to the means of 

 distinguishing sea from land glaciation. Tiiis I believe to 

 be quite possible if careful observations are nuide. Sea 

 glaciation is always accompanied with much smoothing 

 and polishing, and on very hard rocks the striation is 

 comparatively imperfect, while it is usually not quite 

 uniform in direction and often presents two sets of sti'iae. 

 Tiie action of true land glaciers, especially when thick 

 and moving down considerable slopes, produces deep 

 grooves, as well as striae, on vertical as well as horizontal 

 surfaces, and is more fixed and uniform. The more intense 

 forms of sea glaciation, especially if long continued, J^"j, 

 however, approach very closely to the effects of land ice. 



The action of icebergs is undoubtedly, though not the 

 chief, one of the most important manifestations of ice- 



