FORMER GLACIATION OF NORTHEASTERN AMERICA. 381 



the frequent occurrence of striated rock surfaces, such as can only be re- 

 garded as the work of ice. Over an extensive area within the region of the 

 Northern Drift such stria? are common ; and although they have been the 

 object of constant discussion during the past fifty years, unanimity of opinion 

 has not been attained as to whether they are the work of icebergs, local 

 glaciers, or a continental ice sheet. More and more, however, of later j'ears, 

 geologists have inclined to consider these striated surfaces as having been 

 the result of the movement of one confluent mass of ice which advanced from 

 the north, and once covered the whole region now occupied by the Northern 

 Drift. 



This ice sheet was originally considered to have been a part of a general 

 ice-caj), which extended from the Pole in all directions for an indefinite dis- 

 tance, and was regarded as the necessary result of a lowering of the earth's 

 mean temperature. The observations of the last few years have rendered 

 this view of the nature and origin of the ice of the Glacial epoch entirely 

 untenable, as has been abundantly proved in the preceding pages. Most 

 of those who now advocate the former presence of a general ice sheet do 

 not consider themselves bound to indicate whence it came, or imder what 

 topographical or climatic conditions it coidd have originated ; it is sufficient 

 for them that the presence of ice seems to be demanded, and it is, in accord- 

 ance with this necessity, extended over the surface indefinitelj^, regardless of 

 the topography, the conditions of its existence being left to the climatologist 

 to discover. There are those, however, who still incline to look upon the 

 so-called continental glacier with suspicion, and rely chiefly on icebergs 

 floated from the Arctic regions, to which, in conjunction with local glaciers, 

 they ascribe the striation in question. 



There is no doubt that the character of the strife in the Drift region of 

 Northeastern America is extremely perplexing, lacking, as these do almost 

 entirely, that uniformity of direction and radiation from a central area which 

 we have seen to be of so much assistance in makino; out the facts in Scandi- 

 navia. That topographical conditions greatly resembling those now prevail- 

 ing in the Drift region have been largely instrumental in governing the 

 direction of the strife, can hardly be doubted. In the valley of the St. Law- 

 rence, however, and in the vicinity of the Great Lakes generally, the move- 

 ment of the ice, whether fixed or floating, although parallel with the course 

 of the stream, has been in the opposite direction. 



Again, many observers agree in reporting the existence on high ridges 



