﻿GEOLOGY OF THOUSAND ISLANDS REGION 135 



from the Leray to the Lowville. But when increased south 

 dip brings down the Leray again, at the south end of the 

 inlier ? the formation appears as a wall across the valley, and the 

 stream follows the Lowville underground, though its course is 

 marked by a depression in the surface of the Leray above. 

 After flowing underground a short distance the river reappears at 

 the surface, or more strictly the surface comes down to the river 

 level, owing to caving down and removal of the Leray. In 

 plate 38 this emergence of the stream is shown. It quickly passes 

 again underground. The process seems definitely the enlargement 

 of an underground channel by solution until the roof becomes un- 

 supported, sags and caves in where thinnest, with succeeding grad- 

 ual extension of the caving in process, both up and down stream. 

 About Limerick the Leray limestone forming the stream walls 

 is shown in all stages of disturbance due to this undermining pro- 

 cess. The view in plate 23 shows the process in an early stage, 

 and that in plate 39 in a much more advanced stage, the Leray 

 here being in a condition for which Ruedemann's term of 

 " scrambled " is so absolutely applicable, that we can not refrain 

 from utilizing it. 



In plate 40, a view of the stream above the falls at Limerick, 

 we seem to have a direct exposition of what the character of the 

 stream is when underground. It seems distinctly a solution, not a 

 corrasion, channel following the joints in beautiful, zigzag fashion. 

 The chief part of the course shown in the view is on a northwest 

 joint, but in the foreground, and also in the background, it is 

 along a set of north joints. It seems to us highly probable that 

 the stream was formerly underground here. Unquestionably the 

 channel is due to solution along, and guided by, the joints. The 

 locality is so suggestive that it is a pity a longer portion of the 

 stream's course can not be photographed. A contrasting view, that 

 of plate 41, shows a limestone surface (the same limestone) cor- 

 raded and etched by surface solution and wear. 



The influence of the low folds in the Paleozoic rocks in causing 

 falls in the streams which more or less directly flow down the dip, 

 has just been noted in the case of the fall at Limerick. The course 

 of .the Black river across the south margin of the map furnishes a 

 fine illustration of a stream whose fall is precisely that of the dip, 

 and along which, owing to variations in the amount of dip, re- 

 peated falls occur over identically the same rock horizon. The 

 river here has cut a shallow valley in rock, in postglacial times, and 

 the chief falls in this part of its course are at Felts Mills Black 



