﻿GEOLOGY OF THOUSAND ISLANDS REGION 



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set. Figure 4 is an attempt to illustrate these relations. These two 

 joint sets, both having the same strike as the Grenville rocks, are 

 much the most prominent of the joints which these rocks show, and 



Fig. 4 Sketch and section of alternating quartzite and amphibolite bands of Gren- 

 ville series, the quartzites forming low ridges on the surface. The line A represents the 

 direction of the joints which follow the dip, the line B that of those at right angles to the 

 first set, and C represents the direction in which both sets cut the surface 



conspicuous at every good exposure of the Grenville schists or 

 quartzites, though much less conspicuous in the limestones. The 

 quarry face in plate 2 is on the dip joints, here steep, and the other 

 set are quite flat and show well in the view, as does also a vertical 

 set of northwest joints. So common are they that they soon came to 

 be recognized as a matter of course, which it was superfluous 

 to chronicle in the notebook. Hence the number of observations 

 on joints striking n. 40 e.-n. 6o° e. shown on the diagram [fig. 

 5] is misleading as to their abundance and importance. The com- 



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Fig. 5 Diagram to indicate the number of readings on joint directions in the Pre- 

 cambric rocks of the district for each s° point of the compass, the outer row of figures 

 giving this number, and the inner row the compass degrees, corrected for variation 



paratively slight variation in the number of readings for all points 

 between n. 30 e. and east is however a result of, and indication of, 

 the swerving of these joints with swerve in the rock strike. 



