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



granite easily splits ; but these lines rarely exceed three and 

 they are termed respectively " rift," " cut off " and " lift." The 

 latter is generally more or less horizontal; the "rift" is the 

 stronger and the " cut off " the weaker of the two nearly ver- 

 tical lines of splitting. 



During the summer of 1887, as an assistant to Professor N. 

 S. Shaler,* I made a careful study of the jointing and other 

 phenomena exhibited in the granite quarries at Cape Ann,f 

 and in the course of these studies became interested in the 

 phenomenon of " rifting." Later some slides were cut with 

 the idea of determining if possible the nature and cause of this 

 phenomenon. 



The Cape Ann quarries exhibit " rifting " in every form of 

 variation. There are no two quarries in which the u rift" 

 has the same peculiarities and in which it has exactly the 

 same direction. In fact in the same quarry as in the case of 

 the Rockport Granite Company quarry, the rift changes com- 

 pletely in different portions of the large pit. Furthermore, in 

 one case, the "rift" becomes weak and the "cut-off" changes 

 to " rift " in the same quarry. The reason for giving these 

 facts is to show that there is such decided variation that the 

 idea of attempting to explain the phenomena on the theory of 

 widespread and uniform disturbance is out of the question, at 

 least since the granite came into anywhere near its present 

 position. That there has been much faulting in the granite is 

 plain upon the most cursory examination. It might be rea- 

 sonable therefore to suppose that the phenomenon of rifting 

 was produced early in the history of the granite, and that the 

 present variation is chiefly due to faulting and contortion. 



The so-called granite of Cape Ann is a hornblendic granitite 

 containing orthoclase and some plagioclase feldspar, quartz, 

 hornblende, a very little biotite and some magnetite. Under 

 the microscope it is found that the quartz and feldspar show 

 signs of much strain. There are tiny irregular faults and 

 fault breccias (figs. 1, 2 and 3) both in the quartz and feld- 

 spar. With the naked eye the rift can be plainly seen, when 

 it is strongly developed, to be a slightly irregular break cutting 

 the rock and crossing quartz and feldspar alike. "With the 

 petrographical microscope, the crack appears quite irregular, 

 and it can be frequently traced around quartz grains rather 

 than directly across them. The line of breaking will often 

 turn to one side in order to take such an easy path, and when 

 crossing the feldspar it generally, in fact as far as my observa- 

 tions extend, always follows cleavage lines. Macroscopically 



* While much of the detailed work was done by the writer, the general scheme 

 and most of the conclusion must be credited to Prof. N. S. Shaler. 

 f See Xinth Annual Report U. S. Geol. Survey. 



