﻿GEOLOGY OF THOUSAND ISLANDS REGION 115 



in the case of the other formations the great majority of the out- 

 liers are owing to wear on rocks of this folded type. The numerous 

 outliers of Leray limestone on the Theresa and Clayton sheets 

 chiefly mark the positions of basins (points of intersection of 

 synclines of both series of folds), the dips being everywhere in 

 toward the center. Similarly the Lowville inliers which Ruedemann 

 has mapped on the Clayton sheet, north of Threemile and Guffin 

 bays, mark the summit of domes (intersections of anticlines) with 

 dip outwardly from the center. In the case of some of the outliers 

 however, those of the Theresa formation on -the Potsdam west of 

 Theresa for example, the dome structure instead of the basin struc- 

 ture is exhibited, the outlier showing no prominent inface, and with 

 dip outward from the center. The domed structure often shows 

 excellently elsewhere, as for example in the Theresa formation at 

 Orleans Four Corners (Theresa sheet) where the upper surface of 

 a single massive layer of the formation protudes above the soil as a 

 low, shallow dome, dipping outwardly in all directions. Many other 

 examples might be cited and, owing to the abundance of rock ex- 

 posures in the district the evidence of these structures is unusually 

 clear, and it is quite certain that these two sets of low, cross folds 

 occur. 



Postglacial folds. There are in the district at least a half 

 dozen examples of low folds, or buckles, of the surface rocks, which 

 are of very recent origin. Though they form only a minor struc- 

 tural and tomographic feature, they are rather unusual and the in- 

 terest attaching to them is out of all proportion to their size and 

 frequency. The writer has noted three of them in the limestones, 

 Lowville and Pamelia, and Professor Fairchild has called his atten- 

 tion to two others. In addition at least one occurs in the Potsdam 

 sandstone. The limestone folds seem all to conform to a common 

 type so that a description of one of them, and of the one in the 

 Potsdam, will answer every purpose. 



The Potsdam fold occurs 2 miles south of Chippewa Bay, in the 

 northeastern portion of the Alexandria sheet, is near the roadside 

 and easily visible from it. It is 40 yards long, trends n. 28 ° w., and 

 a view of it, taken at the south end, appears in plate 29. It rises 

 sharply from the surface of an extensive plain, underlaid by nearly 

 horizontal sandstone, with but a scanty soil covering and much bare 

 rock exposed. The fold is of bared rock with beautifully glaciated 

 surface, whose striations demonstrate that the buckling has occurred 

 since the glaciation. The central portion is buckled up about 12 feet. 

 The photograph clearly shows that, owing to compression, the rocks 



