﻿134 NEW YORK STATE MUSEUM 



With enlargement of the underground tunnel the roof tends to 

 cave in, at first where thinnest, followed by gradual lengthening. In 

 most cases the cover is thinnest toward the stream mouth and cav- 

 ing in begins there and works slowly upstream. In the case of the 

 creek at Felts Mills, just referred to, the map shows Lowville lime- 

 stone in its bed for a half mile above its mouth, beyond which the 

 Leray forms the bed* rock. In the Lowville for part of its 

 course the stream is above ground, and the point where the f orma- 

 tional contact crosses the stream marks the point of emergence 

 from underground, and the slow upstream working of the roof 

 cave in. In plate 36 is a rather unsatisfactory view of the caved 

 roof of a small stream, unsatisfactory because no position of the 

 camera which looked upstream could be obtained, and we are here 

 merely looking across from one bank to the othjsr, with the nearer 

 bank somewhat hiding the view of the opposite one. The stream is 

 a small one, fed by the underground waters of a Leray prom- 

 ontory of no great extent, but its waters emerge from well down 

 in the Lowville, (which alone appears in the plate) and can be 

 seen in the extreme lower left-hand corner. The caving extends 

 many yards upstream and amounts to some 20 feet in night at 

 the lower end. 



Plate 37 gives an interesting illustration, on a small scale, of 

 another feature. The view shows a Lowville platform, surfaced 

 by a resistant layer of somewhat less solubility, and, on the right, 

 the point of emergence of a small, wet weather stream, flowing in 

 a shallow underground channel in the more soluble material un- 

 derneath. The str am course then curves across the foreground 

 and passes backward and toward the left, its course margined by 

 the projecting edge of the hard layer, which has otherwise been 

 removed from the channel with the exception of the fragment 

 left as a tiny "natural bridge " on the left. 



As already pointed out by Ruedemann, in his account of the Low- 

 ville inliers in the Leray limestone, very interesting under- 

 ground features are shown in the Perch river valley about Limerick 

 (Clayton sheet). The rock structure there seems to us to be anti- 

 clinal, with the Leray limestone at Limerick marking the 

 site of a sag, and the Lowville inlier, just south, the site of a 

 dome of the anticlinal crest. North of Limerick increased south- 

 erly dip transfers the stream from the Lowville to the Leray 

 horizon, south of it diminished south dip transfers the stream back 

 to the Lowville again, the point of transfer being marked by a fall 

 [pi. 23] as is the rule in the streams of the region when passing 



