214 BULLETIN OF THE 



study a dozen or more parts of the field in which the evidence as to 

 the small amount of glacial wearing is particularly clear. These areas 

 are widely scattered, and from the additions which are constantly being 

 made to the list it is evident that they are numerous. While this 

 essay was in preparation, my assistant, Mr. J. B. Woodworth, discov- 

 ered a characteristic field of this nature in the region of hill land, on 

 the western border of Rhode Island, where the decayed schistose rocks, 

 the decomposition of which evidently occurred in pre-glacial times, have 

 not been removed by the action of the ice. At first I was disposed to 

 attribute the absence of erosion in these districts to some local arrest of 

 the ice movement, but a careful inspection of the localities has generally 

 disclosed the existence of areas of hard rock, which bore the normal 

 marks of glacial wearing, showing clearly that the ice moved in the 

 ordinary manner over the area. I therefoi-e felt compelled to frame 

 another hypothesis to account for the arrest of glacial wearing during 

 the greater part of the time when the ice sheet lay over the areas in 

 question. While I am still in much doubt as to the value of these sug- 

 gestions which I have to offer, I may say that they have withstood my 

 own criticisms and those suggested by several of my fellow students of 

 the phenomena for a period of ten years, and it therefore seems well to 

 offer them for more extended debate. 



Hypothesis concerning the Conditions of Continental Glaciers. 



We have already had occasion in the preceding pages incidentally to 

 note the effect of pressure in lowering the freezing point of ice, but it 

 appears to me that we have by no means exhausted the considerations 

 as to the conditions of deep glaciers which are open to us by the im- 

 portant discoveries as to the effects of pressure on ice which were made 

 by the brothers Thomson about forty years ago. I propose, therefore, 

 to review the matter, with the hope of discovering some explanation of 

 the arrest in the wear of the bed rocks which seems to have occurred 

 during the time when a thick ice sheet occupied the northern portion of 

 this continent. There can be no doubt that pressure melting operates 

 in an effective though slight manner even in the superficial portions of 

 an ice mass. The phenomena of regelation exhibited when two bits of 

 frozen water are pressed together, clearly shows the way, as has often 

 been observed, in which the conditions operate, and many other simple 

 experiments serve to indicate an action of the same nature. There now 

 appears to be little doubt in the minds of those who have inquired into 



