26 THE GRANITES OF MAINE. 
the material prior to its crystallization. At one of the North Jay 
({iiarries (see p. 82) a similar parallelism in the mica Occurs, but its 
course is in horizontal waves 20 feet wide and 3 feet high; while at 
another of these quarries similar waves pitch 10°-40°. At the 
Pownal Granite Company's quarry, in Pownal (see p. TDK the rock 
shows a parallelism of the minerals, the planes of structure dipping 
10 . and a thin section of the granite does not -how any bending of 
the mica plates or straining of the quartz particles. 
In some of the Massachusetts and New Hampshire quarries the 
writer found flow structure parallel to the surface' of the granite at 
its contact with the overlying rock and also surrounding and parallel 
to the surface of large blocks of other rock included with the granite. 
(See "Inclusions," p. 52.) 
The very local character of these structural features indicates that 
they are not due to pressure which affected the entire region, but thai 
they originated while the granitic masses were still plastic, for they 
conform to the general direction of the flow or to some local modifi- 
cation of it. A granite that exhibits (low structure i- by some 
writers called a How gneiss. The courses of the line- of this How 
structure in the Maine quarries, when the bands are vertical, are N. 
35° W., N. 20° W., and X. t5° E. 
RIFT AND GRAIN. 
The rift in granite is a feature of considerable scientific interest 
and of much economic importance. It is an obscure microscopic 
foliation — either vertical, or very nearly so, or horizontal -along 
which the granite split- more easily than in any other direction. 
The grain is a foliation in a direction at right angles to this, along 
which the rock splits with a facility second only to that of the frac- 
ture along the rift. After a little experience an observer can detect 
the rift with the unaided eye, where it is marked. The only avail- 
able data on this subject are furnished by Tarr and Whittle." 
Tarr presents four figures reproduced from drawing- made from 
enlarged views of thin sections showing the rift in ("ape Ann 
hornblende-biotite granite. These figures and his descriptions indi- 
cate that rift consists of microscopic fault-, most of which meander 
across feldspar and quartz alike, although some go around the quartz 
particles rather than through them. In the feldspars rift usually 
follows the cleavage. These minute faults are lined with microscopic 
fragments of the mineral they traverse and some of them -end off 
short, minute diagonal fractures on either side. In examining such 
"Tarr, R. S., The phenomena of rifting in granite: Am. Jour. Sci., 3d ser., vol. It. L891, 
pp. 267-272, figs. 1-4: also Economic Geology of the United smirs. 1895, p. 124. 
Whittle, Charles I,., Rifting and grain in granite: Eng. and Min. Jour., vol. To. 1900, 
p. 161, figs. 1, 2. 
