R. S. Tarr — Phenomenon of Rifting in Granite. 271 



it is allowed to fall or is otherwise jarred. In one of these 

 breaks I could plainly trace the passage from a distinct " green 

 seam " to a rift-like break and then into unbroken granite. 



From these facts I have been led to the supposition that 

 before the injection of the dikes, the granite was subjected to 

 contortion and pressure, which finally resulted in the produc- 

 tion of rifting. Contortion would, it seems to me, produce 

 just such a weakness in a hard, brittle, quartz-bearing rock. 

 If, now, along some line of weakness in this partially rifted 

 rock, the cohesion is overcome by the strength of the force 

 which is compressing the mass, a joint plane would be formed. 

 The probabilities are that the joint would be nearly parallel to 

 the general rifted weakness, or else along the other line of 

 weakness approximately at right angles to this. The third or 

 horizontal set of "rift" planes, the so-called "lift" may have 

 been a weakness inherent in the granite on account of the con- 

 traction of the mass during cooling. Subsequent faulting has 

 displaced the granite, and broken it into such a number of 

 pieces that we can no longer trace the general cause of rifting 

 any better than we can that of jointing. 



There is another possible explanation of rifting. There are 

 in Cape Ann several hundred dikes, in many cases extending 

 no doubt completely across the island (see Ninth Annual Rep. 

 U. S. Gr. S., 1887 — Shaler). The injection of this matter has 

 expanded the bed rock several per cent more than its original 

 bulk. This expansion must have been accompanied by a con- 

 dition of great strain, perhaps enough to account for the break- 

 ing of the granite into its present rifted form. As the dikes 

 for the most part follow the prevalent joint planes, these must 

 have existed before the dikes were injected. This fact that 

 the dikes follow the joints would explain the apparent simi- 

 larity in direction of the joint and rift planes ; and the absence 

 of rift planes in certain places could be explained, either by 

 the absence of dikes in that vicinity, or by some local pecu- 

 liarity which relieved the pressure. The one fact which mili- 

 tates against this hypothesis is that there are no signs of rifting, 

 and very little sign of strain in any of the earlier dikes which 

 follow the joint planes, although they must have been sub- 

 jected to great pressure during the injection of the later dikes. 

 Altogether it seems to me more reasonable to suppose that the 

 joints and rift phenomena are due to the same general cause, 

 especially since there is a certain parallelism in direction be- 

 tween the two. 



The inquiry then stands in rather an unsettled condition. 

 The phenomenon of rifting is dependent upon certain micro- 

 scopic breaks and even faults. There is a certain though not 

 very definite parallelism between joints and rift and in some 



