﻿106 NEW YORK STATE MUSEUM 



The foliation of the Laurentian granite gneiss, and of the gneis- 

 soid portion of the Alexandria syenite is concordant with that of 

 the Grenville rocks, and in them these same joint sets are developed, 

 though in a much less prominent way. In the more massive igne- 

 ous rocks they are replaced by a set of vertical, northeast joints.- 



At right angles to the set, or sets, of northeast joints is a set 

 with northwest trend, with planes nearly or quite vertical, and rang- 

 ing from n. 40 w. to n. 55 w. in direction, 27 of the readings 

 falling within those limits. A less conspicuous east-west set is also 

 indicated by the 12 readings between n. 70 w. and n. 8o° w., to- 

 gether with the 14 between n. 8o° e. and e. As seen in the field 

 also this set is more variable and less prominent than the north- 

 west set. The number of northerly readings is not great, and is 

 spread rather uniformly over 50 of compass range, coinciding with 

 the impression given in the field as to the comparative scarcity and 

 great irregularity of that joint set. 



Notwithstanding the rather small number of total readings the 

 diagram shows that 30 out of the possible number of 36 different 

 5 directions are represented. Nowhere in the field were more than 

 four sets of joints noted in a given rock exposure, and all the 

 joints showed considerable tendency to curve and vary in direction, 

 leading to the belief that this spreading of the readings is owing 

 to this variability and in no wise indicative of a great number of 

 joint sets. 



Locally the more rigid of the Precambric rocks, the quartzites 

 and granites, are excessively jointed, the joints being very close 

 spaced, chopping up the rock into small, angular blocks [see pi. 

 3]. In such places signs of slipping are usually to be made out. 

 These so called " shear zones " result from readjustment under com- 

 pression under conditions such that these rigid rocks fractured and 

 slipped along the fractures, while those less rigid, the limestones for 

 example, effected readjustment in other manner. 



That the Precambric rocks were jointed prior to the deposition of 

 the Potsdam sandstone is conclusively shown, firstly by the absence 

 in the Paleozoic rocks of compression joints and shear zones, and 

 secondly by the occurrence of joint cracks in the Grenville lime- 

 stones which became widened by solution and in that condition were 

 filled with sand as the Potsdam sands commenced to be deposited. 

 In the few cases in the district where contacts between Potsdam 

 and Grenville limestone are exposed these features appear [see 

 fig. 1, p. 58] and are apparently widespread. 



