GEOLOGY OF THE VICINITY OF LITTLE FALLS 



49 



irregular, and that in two ways; first, tlie joints are often observed 

 to curve, and second, the various planes of the same system are 

 often far from parallel. Hence the usual imperfect exposures 

 in the woods, which form the larger number of the pre-Cambrian 

 exposures, and which are apt to show only one or two planes of 

 a set, are likely to give widely varying results. 



In many of the pre-Cambrian exposures the two sets of joints 

 shown are, the one parallel to, and the other at right angles to the 

 strike of the rocks. The other exposures show joints which do 

 not conform to the strike, one set making an angle of from 15^ 



S % 9 



Fig. 9 Diagram of 129 readings on joints in the pre-Cambrian rocks. The inner row 

 of figures represents the points of the compass, in degrees from the true north. The 

 outer row gives the number of readings on joints for each compass direction. 



to 45° with it. Sometimes this is brought about by a swerving 

 of the strike while the joints hold their direction, as is the case 

 at Little Falls; at other times it occurs when the strike has re- 

 mained constant in direction. This latter fact seems to the writer 

 to imply perhaps two groups of rather irregular joints; and the 

 diagram, figure 9, would seem to bear out such an interpretation. 

 Bearing in mind that the curving of the joints causes consider- 

 able latitude in the direction of a given set, the diagram shows that 

 the larger number of readings lies between n. 5° e. and n. 25° e. 

 and the next larger number between n. 65° w. and n. 85° w., the 

 two being approximately at right angles. A less well defined 

 group is possibly indicated in the northeast and northwest 

 directions. 



