78 



COUKSES OF STRIDE. 



Fia - 28 - Another evidence of the powerful force 



exerted upon the planished surface is seen, 

 when instead of simple furrows a strip of 

 some breadth exhibits a succession of chips 

 partially started up, as if the abrading agen- 

 cy did not exactly cut its way but moved 

 over the surface with a jarring motion. This 

 is finely shown upon some of the dark lime- 

 stones on the shores of Lake Champlain, and 

 of which Fig. 28 will give an idea. 



We noticed another curious fact on the same specimen. A pebble evidently got 

 loose between the grinding body and' the ledge, and waddled off to one side ; but was 

 soon so completely crushed that all trace of it, as shown on the figure, disappeared. 

 This fact establishes the important conclusion that the striating body was large and 

 unyielding, and had fixed projecting points on its under side, which were made to plough 

 straight furrows. Hence over very uneven surfaces the scratches usually retain their 

 parallelism, even when running obliquely along quite steep slopes. For had the frag- 

 ments which made the grooves been loose, beneath a heavy moving body of any kind 

 they would have waddled off laterally, as in the above example, and consequently the 

 stria? would not have been parallel. 



PREDOMINANT COURSES OF THE STRI^. 



Local variations in the direction of the striae are quite common. But the inquiry has 

 often arisen whether we can discover more than one predominant course, which would 

 indicate any general change in the direction of the drift agency. Connecting the facts 

 brought to light in the Vermont Survey with those previously known of other parts of 

 the country, we venture to point out three predominant directions taken by the force at 

 different times. 



1. FEOM THE NOKTHWEST. 



We might perhaps conclude these striae to have been made the earliest because we 

 find them generally upon the highest mountains. And the force by which they were 

 produced an arctic ocean as we judge then overtopped the mountains bearing icebergs 

 on its surface, which struck against the highest points. As the waters sunk, so that the 

 hills appeared and valleys were formed, the direction of the currents, and consequently 

 of the striae, was modified. So far as we have facts (we wish we had more) the grooving 

 and planishing force (which means with us the direction of the oceanic currents) was 

 turned more towards the east when the country was all beneath the waters, than after- 

 wards. The following courses of the striae on the high ridges of Vermont and Massachu- 

 setts will, we thinkj make this opinion appear probable. 



Mansfield Mountain, summit of the Chin, 4348 feet high,. 

 Jay Peak, 4018 feet high, ..... 

 Camel's Hump, 4083 feet high, 

 Mount Holly, 1415 feet high, 



N. 20 W. 

 N. 40 W. 

 N. 40 W. 

 N. (iO W. 





