50 NEW YORK STATE MUSEUM 



The joints in the pre-Cambrian roclis are vastly better shown at 

 Little Falls than at anj^ other locality on the sheet, the steep 

 joint cliffs in the long railway cuts there being familiar to every- 

 one. There are two conspicuous sets of vertical joints here which 

 are at, or nearly at, right angles to each other (the readings show 

 an angle varying between 70° and 90°) , Both sets vary somewhat 

 in direction; one gives readings of from n. 70° w. to n. 90° w., 

 the other from n. 20° e. to n. 35° e. There is however a thlT'd 

 set to n. 10° w. or thereabouts, which is locally the most con- 

 spicuous of all. The strike of the foliation planes at Little Falls 

 varies between n. 60° w. and n. 90° w., being sometimes parallel 

 with, and sometimes making an angle as high as 40° with the 

 n. 50°-70° w. joint set. This plainly indicates that the variations 

 in direction of joints and foliation are independent of one another. 



In addition to the vertical joints, there are at least two sets of 

 much less steeply inclined joints. These are in the majority of 

 cases dip joints, following closely the direction and inclination 

 of the foliation planes. They are most numerous and pronounced 

 in the Grenville gneisses, but occur frequently in the igneous rocks 

 as well, being specially noteworthy in the granitic gneisses asso- 

 ciated with the Grenville rocks. The other set is at right angles 

 to the first in regard to both strike and dip, and is not so well 

 marked. Both seem to be compression joints, and the fact that 

 the strike of the second set is at right angles to the foliation strike 

 suggests that the two sets are probably due to compressive forces 

 acting at different times and in opposite directions. 



Paleozoic rocks. In these the compression joints are lacking, 

 but the vertical (tension) joints are abundantly developed, and 

 when plotted show the same wide variation in direction found in 

 the pre-Cambrian rocks, so that it is not certain that any set is 

 present in the latter which is not also found in the former. There 

 are however more readings in the direction n. 70° e, than in any 

 other, giving this direction much greater importance than in the 

 pre-Cambrian rocks. 



Since the Paleozoic rocks are folded only in the most gentle 

 fashion, the joints have likely no connection with the folding. 



