R. 8. Tarr — Phenomenon of Rifting in Granite. 269 



the line does not appear as a fault but simply a crack ; but 

 microscopically extremely minute faults can be detected, and 

 the universally present fault breccia proves actual dislocation. 



The fault breccia is in reality a breccia, not as I at one time 

 suspected a secondary development, for in several places I 

 found pieces of feldspar only partially removed from the main 

 4. crystal in such a way as to show the con- 

 ~^-^^_^ tinuation of cleavage lines from the main 

 rrA^Ij%-7 piece into the dislocated portion. Along 

 ^7y^yb the fault lines in the margin of the feld- 

 ■ — spar, cleavage lines are frequently devel- 

 oped, and these tend to hide the fact that dislocation has 

 actually occurred (fig. 4). 



Rifting is, then, dependent upon the thousands of minute 

 dislocations which occur in every cubic inch of rock. The 

 strains which produced these dislocations were of such a 

 nature that three distinct sets were produced. I refer to Pro- 

 fessor Shaler's article on Cape Ann in the Ninth Annual Report 

 of the U. S. Geological Survey for 1887, for a full statement 

 of the joint planes in the Cape Ann quarries. In this state- 

 ment one of the most striking facts brought out, especially in 

 the diagrams is that the joint planes follow distinct systems. 

 There are three of these systems, one nearly horizontal and 

 two vertical. The horizontal system is probably a contraction 

 jointing formed during the cooling of the granite. These 

 joints are irregular both in position and dip, are usually more 

 or less dome-shaped, and lie one beneath another in con- 

 centric layers. The vertical sets of joints are much more 

 regular, and are frequently perfectly straight cracks for several 

 hundred feet horizontally and more than fifty feet vertically. 

 They form with each other an angle several degrees less than 

 a right angle. In almost any quarry these systems of jointing 

 may be readily detected. Other joints occur, striking in 

 almost every direction ; but they are clearly accidental being 

 most commonly diagonal across a block formed by the main 

 joint planes, while the two main sets of vertical joints are the 



