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NEW YORK STATE MUSEUM 



great igneous masses, a nearly horizontal set also appears, but 

 this is more intermittent and less regular than the others. 



Since the usual foliation strikes of the region are either to the 

 northeast or to the northwest, it is likely that the inclined joints 

 in those directions represent compression joints in the shearing 

 planes, these being more or less controlled by the foliation. An 

 instance of the sort is illustrated in plate 15. The cliff there 

 shown is a joint cliff, with a n. 50° w. direction, an important 

 joint direction in the vicinity. Two sets of inclined joints cut the 

 face of the cliff, both of which have a strike of n. 30° e. The one 

 has a fairly uniform hade of 35° to the northwest, while the other 

 is much more irregular, often swerving into a horizontal position, 

 but in general hading to the southeast. The rock is augite syenite, 

 with a very rude and imperfect foliation, which strikes about 

 n. 40° e., closely approximating the strike of the joints. At the 

 right of the view another joint set appears with a n. 70° e. strike 

 and a hade of 15° to the south, and there is yet another nearly 

 vertical set, not appearing in the view, which has a n. 20° w. 

 strike. It would seem very likely here that the n. 30° e. joints 

 are compression joints, produced in the shearing planes. 



Such instances as the above are rather exceptional, however, 

 and the usual, nearly vertical joints which prevail throughout the 

 region have not yet been successfully classified. 1 That at least 

 an east-west system had been developed in Precambric times 

 is indicated by the prevalence of that trend in the diabase dikes. 

 They vary from it through 20° or 30° both to the north and 

 south, but within those limits have it so uniformly as to indicate 

 not only the presence of a fissure system with this trend, but 

 also that this set constituted the line of least resistance to the 

 upward movement of the molten rock. This might have been 

 because this was the only, or the best developed set, but move 

 likely the use of it to the exclusion of other sets was determined 

 by the direction of the side pressure which prevailed at the 

 time. 



That minor faulting has often occurred along these joints has 

 already been set forth. It has not yet been determined whether 



1 Such for example as the n. 50° w. series of plate 15, well shown also in 

 plate 16, a nearly vertical set which is quite persistent over a very large 

 area in the mid-Adironclack region. 



