48 NEW YORK STATE MUSEUM 



Where the strike swerves to the northwest from its usual direc- 

 tion, the dip is usually found to be to the north, and a correspond- 

 ing change to the northeast is accompanied by a south dip. 

 Usually a change in dip from north to south is accompanied by a 

 swerving of the strike as above noted, though, where the dips are 

 gentle, the change is apt to be very slight. But, so far as it goes, 

 the evidence ii;idicates a general pitch of the folds to the east. 



Joints 

 Pre-Cambrian rocks. The pre-Cambrian rocks are invariably 

 much jointed. The larger number of the joints are vertical, or 

 nearly so, though they may depart from the perpendicular by 

 varying amounts up to as much as 30°. It is an exceedingh' diffi- 

 cult matter to reduce these joints to any sj'stem, since they show 

 a surprising lack of uniformity in direction. Most individual ex- 

 posures show vertical joints in only two directions, though some- 

 times a third, and rarely a fourth is added. While these two direc- 

 tions are tolerably constant locally, ±hej vary widely from place 

 to i)lace. A large numbe'r of readings have been taken- on these 

 joints, and, when it is considered that the area on the map occu- 

 pied by these rocks is only some 50 square miles, the great varia- 

 tion that they show in direction is surprising, and it seems almost 

 futile to attempt to reduce them to an}^ system. To illustrate, 129 

 readings on these joints were so selected as to represent rather 

 uniformly the pre-Cambrian area, the readings rejected being 

 some of those from places where, because of frequent outcrops, 

 many more than the average number were available. These were 

 plotted as shown in figure 9. In general, readings can not be 

 taken closer than within 5° ; and all others have been plotted at 

 the nearest 5° point (33° being made 35° and so on). On this 

 basis there are 3C> possible directions of joint planes, and out of 

 these 31 actually occur. Were the joint planes regular in direc- 

 tion, this would imply a great number of joint systems, but the 

 diagram is itself prima facie evidence that they are not regular. 

 Moreover at most outcrops but two systems are to be seen, and 

 also at most outcrops one or both sets are actually seen to be very 



