50 J. D. Dana on the origin of some of the 



trap Mils. Some deviation from the general course would take place 

 wherever there are high ridges or deep valleys varying in direction 

 but little from the main course of the movement, just as a deej) 

 trough in the bottom of a stream of water set a little obliquely to 

 the current would deflect the waters and give them more or less 

 nearly its own course ;* and this is what Percival observed in the val- 

 ley between the Hanging Hills of Meriden and Lamentation Mountain. 

 In East Haven, on the eastern border of the New Haven region, the 

 direction is S. 13° E. The facts sustain the conclusion that the gene- 

 ral course was that of the Connecticut river valley. To the westward 

 of the central portion of the valley, over the eastern Green Mountain 

 slope, the general course, as various observers have shown, is to the 

 east of south, or about south-southeast, which is a natural resultant of 

 the two forces — that producing the main southerly movement, and 

 that arising from the eastward, or E. by S. slope of the surface. 



As the slope southward was very small compared with that in the 

 Alps, the motion was much slower — probably not exceeding a mile in 

 a century, which is equivalent to about a foot a week. The move- 

 ment was not continuous at this rate, but by starts, at longer or short- 

 er intervals — weeks, months or years — as the resistance could be over- 

 come. Having a thickness to the north of more than four thousand 

 feet, the presbure it exerted wherever its lower surface rested was 

 enormous, and when it did move the abrasion was commensurate with 

 it. It was not, like an Alpine glacier, confined between the sloping 

 sides of a valley, the declivities of which aided largely in its support, 

 and so relieved the bottom partly from pressure ; it lay spread out 

 over the plains and hills resting heavily upon the most of the surface 

 beneath it. 



The movement produced three results, as has been well illustrated 

 by the principal authorities with regard to glaciers. First, a break- 

 ing of the brittle ice wherever there was friction, resulting in opening 

 immense crevasses where the resistance was great (for the gla- 

 cier owes its power of movement to the facility with which it breaks 

 and mends itself ) ; secondly, the abrasion of the rocks beneath, re- 

 sulting in a ploughing out of the soft half-hardened sandstone to 



* In the case of large continental valleys, the glacier followed the course of the val- 

 ley even when this course was east-and-west, as is shown by the author to have been 

 true of the Mohawk valley, in his Geology (p. 751), and in the American Journal of 

 Science, [2], vol. xxxv, p. 243, and by Dr. Newberry, with reference to other regions, 

 in the paper referred to on the preceding page. 



