270 R. S. Tarr — Phenomenon of Rifting in Granite. 



result of one general cause, and in consequence follow a gen- 

 eral law. In different portions of the Cape, the joints extend 

 in different directions, but these variations may be readily ex- 

 plained on the supposition of faulting which has clearly been 

 extremely common in this granite mass. In a number of cases 

 the rift is found to follow the same general direction as the 

 principal set of joints. This is particularly the case where the 

 rift is well developed, as for instance in the quarries at Bay 

 View. 



In regard to the age of the rift, I can say very little that is 

 definite. A careful study of 70 slides, cut from specimens of 

 rock taken from the diabase dikes of Cape Ann, failed to show 

 any sign of rift. From this it would appear that the rifting 

 tendency was induced before the dikes were injected into the 

 granite, but as the dikes mainly follow the joint planes, this 

 is of little value in placing the age of the disturbance. There 

 are some ragged dikes of quartz porphyry which were intruded 

 bpfore the jointing, but these show no signs of rifting. The 

 evidence furnished by the dikes, however, is of little value for 

 line-grained rocks would not be likely to show evidence of 

 microscopic faults. This fact is particularly well seen in the 

 Cape Ann granite, in places where there are fine grained basic 

 segregations. . These segregations are as old as the granite, yet 

 they show no rifting, although the surrounding granite has a 

 very strong rift. The older dikes of quartz porphyries, and 

 ragged diorite are very much faulted macroscopically, and very 

 much changed in mineralogical composition microscopically, 

 showing that there has been much motion and consequent 

 alteration. 



I suspect, although no evidence that I have been able to find 

 conclusively proves it, that rifting and jointing are closely 

 associated phenomena, and that the cause which produced one 

 caused the other. What this cause was I shall not attempt to 

 say, though plainly it was one of great extent and vast force. 

 Between the almost microscopic rift and the joint plane 

 hundreds of feet in linear extension, I have been unable to 

 find any satisfactory gradation. Even the similarity in direc- 

 tion is not as striking as could be desired, to prove the connec- 

 tion. At Bay Yiew, however, where the rift is remarkably 

 strong, in some exposed places it has doveloped into many 

 small parallel joints. These joints are about six inches apart; 

 but if the rock is struck with a hammer it cleaves into many 

 pieces parallel to these rift joints. Certain so-called "green 

 seams " have been observed to pass out into rift-like breaks. 

 These " green seams " are imperfect joints, lined with a thin 

 layer of chloritic matter, which are not generally noticed by 

 the quarrymen ; but along which the rock is liable to split if 



