﻿104 NEW YORK STATE MUSEUM 



into two pairs, each pair consisting of two joint sets at right 

 angles to one another, and the joints of one pair bisecting the 

 angles between the joints of the other pair. In such districts it 

 is seldom the case that all four joint sets are exhibited in a 

 single rock exposure, two or perhaps three of the four showing, 

 rather than the whole number. In many, if not in most, regions 

 where four or more joint sets occur, it is found that one pair 

 tends to north-south and east-west directions, with another pair 

 showing northeast and northwest trends. The joint planes often 

 curve somewhat, so that the compass direction of a given set may 

 vary through a considerable number of degrees. This tendency 

 much increases the difficulty of discrimination between the dif- 

 ferent sets in districts where more than four are present, as is 

 quite frequently the case. 



In folded rocks the character of the jointing differs considerably 

 from that found in rocks not folded. Since in our region here we 

 have rock masses of each sort, Precambric rocks which have been 

 greatly compressed and f olded, and overlying Paleozoic rocks which 

 are comparatively undisturbed, it will be convenient to consider 

 them separately. 



In the Precambric rocks. The diagram [fig. 5] presents a 

 summation of the readings taken on the joints of the Precambric 

 rocks of the district included in the maps. They are comparatively 

 few in number, partly because of the comparatively small area 

 which presents these rocks at the surface, and partly because the 

 joints were found to be so irregular that no satisfactory readings 

 could be obtained in many exposures. The rocks are not as abun- 

 dantly jointed, nor are the joints as clear-cut as usual in the Adiron- 

 dack region. 



In closely folded sediments, such as the Grenville, joints are apt 

 to be present as a result of compression, and to have their direc- 

 tions controlled to a considerable extent by the folds, or in other 

 words by the strike and dip of the folded sediments. These have 

 been shown to have a general northeast strike throughout the dis- 

 trict, though locally varying in direction through more than 90 . 

 The more usual direction however is n. 40 ° e.-n. 6o° e. Two sets of 

 joints are present which have the same surface trend, that of the 

 rock strike, the one set controlled by the dip and having approxi- 

 mately the same inclination, the other inclined in the opposite direc- 

 tion, or to the southwest, and closely at right angles to the first 



