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NEW YORK STATE MUSEUM 



and the two sets cutting one another at 45°. It is to be noted 

 that while the north and east joints of the first pair are at 45° 

 to the strike of the foliation planes, the northeast joints of the 

 second set are parallel to and the northwest at right angles to 

 these planes. Whereas therefore this latter pair of joints might 

 have been produced by forces acting in the same direction as 

 those which caused the foliation, the first pair could not have been 

 so produced. It should also be noted that the first pair are more 

 prominent in the Paleozoic rocks than are the second, while they 

 p.re about equally developed in the pre-Cambrian rocks. 



Many joints have been noted which can not be brought into 

 either of these sets, but they are of much less importance and 

 their significance is not clear. The numerous tension joints of 

 the later dikes, which are manifestly due to contraction on cool- 

 ing, are not here had in mind. 



Wherever observations on the dip of the n 20° w and n 70° e 

 pair of joints have been possible, they have been found to be ver- 

 tical or very steeply inclined. But the northeast joints, those strik- 

 ing with the foliation planes, have been several times noted with 

 dips of from 45° to 60° to both the southeast and the northwest, 

 indicating that they are compression joints, and that the pressure 

 came from the same direction as the earlier pressure which pro- 

 duced the foliation. In general these joint planes do not coincide 

 with the foliation planes but are somewhat more steeply inclined. 

 Observations on the dip of the northwest joints are practically 

 lacking, though they seem to belong with the previous set and to 

 have been produced at the same time. The vertical north and 

 east joints are perhaps tension joints. 



As the compression joints are lacking in the Paleozoic rocks, 

 they must antedate the Potsdam. The prevailing east and west 

 trend of the diabase and syenite dikes argues for an existing set 

 of fissures with that trend at that time. The Palezoic rocks are 

 also jointed, mainly by the north and east set. At least three, 

 and probably several periods of joint formation are thus indicated. 



Many of the joints are slickensided, indicating that slight slips 

 have taken place along them, and not improbably a considerable 



