BOCK VARIATIONS. 43 
thickness they range from a fraction of an inch to 6 feet, but usually 
from 2 inches to 2 feet. 
The courses of 16 aplite dikes are distributed as follows : 
Courses of 16 apliU dikes. 
X.-X. 10° E 3 
X. 25° E 1 
N. 60°-77° E 2 
X. 10°-30° W 4 
X. 4o° W 1 
X. 60°-80° W ___ 5 
Dikes that strike in the uorthwesterly-southeasterly quadrants are 
most numerous. 
In color these dikes vary from bluish gray to light and dark red- 
dish. The texture of some aplites is so fine that the mineral particles 
can not be distinguished with the unaided eve: that of others is 
so coarse that the feldspar and mica may be thus detected. Under 
the microscope the dimensions of the particles range from 0.05 to 0.75 
mm., the average being about 0.16 mm. for the finer ones and 0.50 mm. 
for the coarser ones. Some aplites have a porphyritic texture. 
Two typical aplites will be described in detail. One, from the John 
L. Goss quarry, on Moose Island, near Stonington. is from a dike 15 
inches wide and over 200 feet long, consists largely of quartz, potash 
feldspar (microcline), and a soda-lime feldspar (oligoclase) in parti- 
cles ranging from 0.047 to 0.141 mm. in diameter, a few thinly dis- 
seminated particles of the same minerals measuring from 0.55 to 1.45 
mm. and a few scales of black mica measuring up to 0.47 mm. An- 
other aplite, from the Sands quarry, at Vinalhaven, consists mostly of 
quartz, but contains some potash feldspar (orthoclase and micro- 
cline), still less soda-lime feldspar (oligoclase), and a few scales of 
black mica. The particles range from 0.047 to 0.3 mm. in diameter. 
The minerals of aplite dikes are so firmly attached to the granite 
on either side that the rock readily splits across both granite and 
aplite. Under the microscope the minerals of the dike appear to be 
welded, so to speak, to those of the granite. In construction the 
blocks containing such dikes should not therefore necessarily be 
regarded as places of weakness, but in a quarry at Franklin, Hancock 
County, the granite is close jointed for a space of a foot on either side 
of an aplite dike, the joints being parallel to the dike. 
At the Bodwell Granite Company's quarry (see p. 168), 2 miles east 
of Jonesboro, Washington County, the reddish granite is traversed by 
a 6-foot dike of rather coarse, dark-reddish aplite, in which the higher 
power of the microscope shows that the source of the color lies in 
exceedingly minute dots of hematite. The aplite contains also musco- 
vite, biotite, and accessory pyrite. This dike crosses the quarry in a 
N. 20° W. direction. A similar dike, having a like course, but only 
