﻿GEOLOGY OF THOUSAND ISLANDS REGION 



107 



In the Paleozoic rocks. In the Paleozoic Rocks of the district, 

 Potsdam to Trenton, the joints are vertical, or nearly so, and show 

 also considerable variability in direction, though this seems not quite 

 so pronounced as is the case in the Precambric. Figure 6 gives 

 a diagrammatic summary of 280 readings on these joints, and shows 

 again a spreading to all points of the compass, 34 of the 36 possible 



,<#: v 4%> 



Fig. 6 Diagram, similar to that of the preceding figure, of the joints of the Paleozoic 

 rocks 



directions being represented. The great number of readings in the 

 direction n. 70 e.-n. 8o° e. constitutes the most prominent feature. 

 The next point at which readings are concentrated is the n. 50 w. 

 direction, but readings with this general trend are spread from 

 n. 40 w. to n. 65 ° w., in other words these joints are somewhat 

 less true in direction than those of the preceding set, which may 

 however be regarded as extending from n. 60 ° e. to n. 8o° e. A 

 third direction of more abundant readings, from n. 20 e. to n. 40 e. 

 is also sihown, while the fourth direction, n. io° w.-n;. 30 w. is the 

 least prominent of all. This last, however, is the one at right angles 

 to the first, and most prominent, set. As thus outlined there are 

 99 readings for the first set, 81 for the second, 45 for the third and 

 but 25 for the fourth. There remain 40 readings which lie wholly 

 without these groups. It is to be noted that the mean directions of 

 the four groups do not correspond with the cardinal points of the 

 compass, but show a general deviation of 20 from them. 



In the field the majority of the exposures exhibit but two good' 

 joint sets, though usually a third quite irregular set is present. With 

 two good sets shown it is the exception that they are at right angles, 

 and it is the east-west set and either the northeast or the northwest 

 set with it, that usually appear. Often all three of these sets appear 

 with lack of only the north-south set, and with the east- west set 

 customarily the most prominent and regular. On bared rock sur- 



