42 THE GR4NITES OF MAINE. 
a joint striking N 25° E. has subjoints striking N. 40° E., N. 00° E., 
and N. 50° W. 
Woodworth has studied analogous and related structures in various 
rocks and has described them as w * joint fringe " and " feather 
fractures."" 
CONTEMPORARY FRACTURES. 
Recent natural fractures have occurred, so far as known to the 
writer, at only three places in Maine granite quarries. One of these 
fractures, already referred to in the discussion of the origin of sheet 
structure (p. 34; see also quarry description, p. 155), occurred at the 
Mount Waldo quarry, near Frankfort. Here the course of the frac- 
tures ran from north-northwest to south-southeast through the center 
of the quarry for a distance between 200 and 300 feet. The sheets 
removed from that part of the quarry aggregated aboul 20 feet in 
thickness. The fracture was vertical and parallel to the (low struc- 
ture, but at right angles to both sheets and rift and at an angle of 
25° to the strike of the grain. The other two fractures occurred at 
the Tayntor quarry, near Ilallowell. Their course was east-west. 
One of these, which was 40 feet long and vertical, passed across a 
horizontal sheet 4 feet 6 inches thick, extending diagonally between 
two channel cuts that formed a right angle Here the rift is hori- 
zontal, a faint vertical grain structure strikes X. To W\, and a 
vertical flow structure strikes X. 35° W. At Hooper, Ilavev & Com- 
pany's quarry, in North Sullivan, the rock is under a compressive 
east-west strain, as it tends to fracture north-south across the grain 
and rift. At many Maine quarries the horizontal movement of the 
rock crushes the " cores" left between adjacent drill holes in making 
" channels." 
ROCK VARIATIONS. 
Under the term " rock variations " are grouped all those varia- 
tions from typical granite that are due to injection, segregation, 
infiltration, inclusion, and steam cavities. 
DIKES (GRANITIC). 
The granitic dikes in the Maine quarries are of three kinds: 
Extremely fine grained granite (aplite), very coarse grained granite 
(pegmatite), and fine or medium grained granite. 
Aplite differs from ordinary granite by the greater fineness of its 
texture and its scant content of mica. Tt is known by quarrymen as 
"salt horse" or "white horse." The courses of these dikes at each 
quarry are given in the diagrams or descriptions in Part II. In 
" Woodworth, J. I'.., On the fracture system of joints, with remarks <>n certain great 
fractures: Proc. Boston Soc. Nat. Hist., vol. 27, 1896, |>|>. L69-173, pis. 1, 2. 
