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



cases the rift has actually developed into a joint. These facts 

 indicate, though rather indefinitely, a common origin for the 

 two phenomena though it is not impossible that the rifting 

 phenomenon is subsequent in origin to the time of formation 

 of the joint planes and possibly the result of the intrusion of 

 the great number of dikes which cut the granite base of Cape 

 Ann, and which must have brought about a condition of intense 

 strain in the mass. A careful study both in the field and the 

 laboratory fails to bring about a definite and satisfactory expla- 

 nation of the cause of the phenomenon ; and its settlement 

 must await studies in other regions where dikes are less abund- 

 ant. 



Aside from the economic value of the rifting tendency there 

 is a geologic effect of considerable importance dependent upon 

 it. It was noticed in the study of the Cape Ann region that 

 certain of the bowlders in the morainic drift had a tendency to 

 crumble while others had no such tendency. This crumbling 

 has gone so far that glacial bowlders which at the time of their 

 transportation must have been solid and in a measure fresh, 

 have, since the end of the glacial period, completely decayed 

 to a crumbling mass of gravel. All stages in this decay may 

 be seen on the island of Cape Ann. At first it was thought 

 that this was the result of some chemical weakness in the rock, 

 but a microscopic examination proved that the weakness was 

 not chemical bnt mechanical. Whenever the rift is strongly 

 developed the rock has the tendency to decay along the rift 

 breaks ; and at places, notably at the Bay Yiew Quarry along 

 the line of the railway in a fresh cut not more than fifteen 

 years old, the decay along the line of the rift has not only 

 developed well marked joint-planes but has begun to crumble 

 the granite into gravel. A better case than this even is illus- 

 trated in the photograph (Plate LI, 9th Annual U. S. G. S.)> 

 where several large bowlders of degradation have resulted 

 through the agency of the weather acting along the rift planes 

 in post glacial times. Thus the phenomenon of rifting is, in 

 the Cape Ann region at least, an important geological agent 

 because of the aid which the lines of mechanical weakness 

 furnish the agents of disintegration. Prior to our investiga- 

 tions, so far as I know, no notice has ever been taken of the 

 phenomenon of the rift, yet I have no doubt it will be found 

 to be of equal importance in many other localities than the 

 one studied. 



