412 NEW YORK STATE MUSEUM 



of compression, as is apt to be tbe case, and this also argues for 

 the late Paleozoic date of the faulting. 



Joints 



All the rocks of the region are cut by joints, but they are spe- 

 cially abundant in the Precambric rocks. Since joints can be 

 formed only in, the zone of fracture, their development in these 

 rocks must have been long subsequent to the formation of folds 

 and of foliation, during which interval the rocks had been ap- 

 proaching the surface because of wearing away of what was above. 

 The joints which are found in the Precambric rocks and not in 

 the overlying paleozoics must be of Precambric age. 



Joints may be produced either by coniipression or by tension. 

 Those of the latter sort are usually vertical or nearly so, while 

 those of the former m>ay be either vertical or inclined. The sim- 

 plest case of tension jointing is the production of columnar joint 

 cracks in igneous rocks owing to contraction on cooling. Some of 

 the jodnts of the dikes of the region are of this class, it being the 

 invariable experience that they are more excessively jointed than 

 are the inclosing rocks. The solidifying and cooling of the great 

 igneous masses of the region, however, took place at sach great 

 depths as to be below the zone of fracture, and hence they lack 

 joints of this character, being neither more nor less jointed than 

 are the neighboring»gneisses. 



Tension joints may also be produced by the desiccation of 

 marine sediments underground, and this cause may have operated 

 somewhat in the production of joints in the Paleozoic rocks, 

 though it is doubtful. 



Another very likely cause of the prodiuotion of tension joints 

 is the slow reduction of temperature brought about in ;rock m!a;sses 

 as they approach the surface beicause of the slow removal by 

 erosion of the overlying rocks. Though the process is an exceed- 

 ingly slow one, and the changes of temperature involved are not 

 large, yet, considering the great areal extent of the rocks con- 

 jcerned, the necessary contraction would seem considerable, and 

 likely to much exceed the elastic limit of the rocks. 



Where rocks are folded in a complex manner, as is usuall}'^ the 

 case, toirsional effects are siure to be produced, which result in the 

 prodnction of two sets of joints, one running parallel to the axes 

 of the major, and the other to the axes of the minor folds. 



