GEOLOGY OF THE LONG LAKE QUADRANGLE 487 



This grouping still leaves a large number of not included joint 

 readings, especially in the n. 20 e. and n. 70 e. directions. This is 

 either indicative of two additional sets of joint couples, a n. 20 

 e. to n. 70 w., and an. 70 e. to n. 20 w. couple, or else shows 

 that the amplitude of swerve in the original couples is consider- 

 ably more than 20 . If it be as great as 40 the extreme directions 

 of swerve of adjacent pairs would meet, or overlap. But it is 

 very improbable that the amount is as great as this. 



If the region consists of faulted blocks, as is quite likely, and if 

 the joints are older than the faults, also highly probable, then a 

 reasonable and probable explanation of the apparent confusion 

 would be furnished. Both hade and throw vary along faults, caus- 

 ing some change in horizontal orientation in the various fault 

 blocks, which may at times become considerable, and produce an 

 equivalent shifting in the directions of preexisting joints. Hence 

 the prevailing joints in adjacent fault blocks might well show a 

 lack of accord in direction, thus accounting for the prevalence of 

 certain joints in certain districts and their absence elsewhere. For 

 example in the southwestern part of the quadrangle the more com- 

 mon joint directions are n. io° e. and n. 50 e., the latter set more 

 variable in direction than the former. The set at right angles to 

 the first is still more variable, from n. 8o° e. to n. 8o° w. in direc- 

 tion, wmile the northwest set is most variable of all, and happens to 

 be the strike joint set. In the southeast the n. io° w. direction is 

 the most prominent, there are no n. io° e. joints and but one read- 

 ing to n. 50 e., the two prominent directions in the southwest. 

 The n. 8o° e. to e. and w. direction is next in prominence and is the 

 strike joint set. The northwest * set is again very variable in 

 direction. 



In the fairly massive eruptives, where there is little" or no folia- 

 tion, the joints are mainly highly inclined to vertical. Hades up to 

 20 from the vertical are common, especially in the curving joints. 

 But there is often present a set of nearly horizontal joints, also 

 quite irregular. 



In the gneisses of the southern half of the quadrangle there is a 

 joint set which is plainly dependent upon the foliation. This 

 varies in general from an. 8o° e. to a n. 50 w. strike, and in many 

 exposures good dip joints are seen whoss strike is nearly or abso- 

 lutelv identical with that of the foliation. There are seen also to 



