STRUCTURE OF MAINE GRANITES. 29 
Courses of grain. 
Number of quarries. 
N 2 
N. 20°-25° W 2 
N. 45°-75° E 4 
N. 45°-72° W 6 
E.-W. to N. 80° E 10 
It appears, therefore, that when rift or grain is vertical the east- 
west and west-northwest to northwest courses are the most common, 
and next the north and east-northeast to northeast courses. 
Rift and grain are not always pronounced. Either or both may be 
very feeble or may be absent. At some of the Redbeach quarries, 
owing to the absence of both, it is difficult to hammer out even a good 
hand specimen. 
At the Armbrust quarry, in Vinalhaven, there is a horizontal rift 
confined to a 4-foot mass striking across the hill with a N. 65° W. 
course. 
The presence of fairly good rift or grain is an important economic 
factor in the granite industry, for it diminishes both the amount of 
labor in drilling for blasts and in splitting. 
The cause of rift structure and the relative time of its formation 
are not yet known. If rift were always vertical it might -appear to 
be closely related to those joints which are nearly parallel to it; but 
that would not explain the rift when horizontal, and horizontal rift 
can not be related to the sheets, which it intersects at various angles 
in many granites. In general it is evident that since it crystallized 
granite has been subjected to strains that have caused either two sets 
of vertical microscopic fractures extending at right angles to each 
other, one more pronounced than the other, or one set of similar hori- 
zontal fractures crossed by a vertical set. 
That the rift is a factor in the crushing strength of granite is 
shown by the results of tests of Mount Waldo granite from Frank- 
fort, blocks of which when placed on the bed — that is, with pressure 
applied at right angles to the rift — showed an ultimate strength to 
the square inch of 31,782 to 32,635 pounds (average, 32,208), but 
when placed on the side — that is, with pressure applied parallel to 
the rift — showed an ultimate strength of from 29,183 to 30,197 
pounds (average, 29,690). In the first test the first crack appeared 
in the block at a pressure of from 120,000 to 123,300 pounds (average, 
121,650) and in the second test it appeared at one of 107,400 to 
112,600 pounds (average, 110,000 pounds). a 
"Reilly, J. W., Ordnance Kept., tests of materials, etc. (1900), 1901, p. 1119. 
