GEOLOGY OP THE NORTHERN ADIRONDACK REGION 413 



Compression of rocks in the zone of fracture may give rise to 

 jointing which follows the shearing planes. There would be two 

 different joint sets, which would cut the surface parallel to each 

 other. Such joints would be inclined instead of vertical, the 

 amount of inclination depending on how largely the shear was de- 

 termined in direction by preexisting planes of weakness, such as 

 bedding or foliation planes. In simple folding both sets of such 

 joints would be parallel with the strike of the beds, one dipping 

 with, and one against them. 



The Adirondack Precambric rocks are much jointed. It is 

 however a difficult region in which to obtain accurate measure- 

 ments of the hade of the joints, though observations on the strike 

 are easily obtained. 1 Moreover, in much of the district the rocks 

 are igneous and poorly foliated, baffling any efforts to determine 

 the structural significance of the joints. The writer's observa- 

 tions have been mainly made in such districts and are not yet 

 sufficiently extended and worked out. Certain things are how- 

 ever clear. 



The Precambric rocks are much more conspicuously jointed 

 than are the overlying paleozoics, implying a time of joint forma- 

 tion prior to the deposition of the latter. 



Joints are not equally conspicuous in all of the Precambric 

 rocks, being least prominent in the limestones, and most so in the 

 great igneous masses, implying some joint formation while the 

 rocks were at sufficient depth to render the weak limestones some- 

 what plastic, though the igneous rocks were thoroughly rigid. 



Four sets of joints are usually to be made out in the Precam- 

 bric rocks, though all four are seldom present in any given 

 exposure. Though varying considerably in direction from place 

 to place, they can be apparently referred to two main sets, the one 

 consisting of a pair of north-south and east-west joints and the 

 pther of a pair of northeast and northwest joints, both sets 

 swerving in direction through 15° or 20°. In some exposures one 

 set is the more prominent, in some the other set ; in many at least 

 three of the four show, and not infrequently all four. The north 

 and east joints are usually vertical, or nearly so, while the others 

 frequently show a greater hade. Not uncommonly, specially in the 



*As in the case of a fault, the hade is the angle of inclination from the 

 vertical. 



