250 SURFACE GEOLOGY. 



porphyritic gneiss than this, and they fall off precipitously for 200 feet. The mortar is 

 2h feet from the edge of the rock. It is not perfect, as a notch is gone on the south 

 side, so that the cavity is shaped like a pitcher with the nose broken off. The width 

 across the top is i foot 3 inches ; depth, on the north side, 2^ feet ; depth below the 

 notch on the south side, li feet. The bottom is rounded, and it is frequently filled 

 with water. 



Mr. Upham has described the pot-holes of the Orange and Newbury summits, and 

 Warwick, Mass., on pages 63-66. 



At Amoskeag falls, Manchester, on the Merrimack, are several pot-holes ; one of 

 them is 12 feet in diameter and 25 deep. 



The "Purgatory," on the line of Lyndeborough and Mont Vernon, shows other pot- 

 holes. The stream — 10 feet wide — winds spirally through a narrow chasm, and then 

 falls 15 or 20 feet into a large pool. Both the spiral course and the pool are to be 

 classed with pot-holes, and there are several small examples above the main cataract. 

 A cave below is thought to have originated from ordinary disintegration through 

 freezing. 



Local Movements in Vermont. Besides the tributaries of the Con- 

 necticut glacier ah-eady mentioned, I will speak briefly of the other val- 

 ley movements occurring in the area of our majD. There has been an 

 easterly movement up the valleys of the Lamoille, Missisco, and Winoos- 

 ki, perfectly distinct from the general south-east progress of the conti- 

 nental sheet. It seems to have been a modification of the south-east 

 current, caused by the contour of the country, and in no sense a local 

 glacier. They are to be compared with the movement of the ice up the 

 valley of the Peabody river (p. 208). 



I found excellent local glacier phenomena in the valley of Lincoln 

 brook, a tributary of Mad river in Warren. In Plainfield, boulders of 

 granite seem to have been transported north-westerly to the main village 

 from Harris and Goshen gores. Other cases are probably Trout brook 

 in Montgomery, and near Lake Memphremagog from the west. 



Lake Basins. 



One of the evidences of glacial action is seen in the hollowing out of 

 basins in the solid rock, which are now occupied by lakes or ponds of 

 fresh water. If one travels around the border of certain lakes, he will 

 discover that it is everywhere rock-bound. In considering the possible 

 methods of excavation of such a basin, some cannot be accounted for 

 except by glacial action. We should consider, first, whether it might be 



