44 THE GRANITES OF MAINE. 
4 feet wide, occurs at the eastern end of the quarry. A third dike, 
which ranges in width from 3 to 6 inches, has a X. 75°-80° E. course, 
and a fourth, of fine-grained material, from one-half to 1 inch wide, 
crosses the others with a course X. 60° W. and can be traced for 200 to 
300 feet. This is evidently of later date than the others. Aplite dikes 
are supposed to have originated in the same deep-seated molten mass 
as the granite they traverse, but they represent a later stage of igneous 
activity. The fissures they till were the result of various tensional 
strains or contractions, possibly consequent upon the cooling of the 
granite. 
Most aplites contain a slightly higher percentage of silica than 
granite. Five analyses of aplites from the Far West made at the 
laboratory of the United States Geological Survey" show a range of 
silica from 71.62 to 76.03 per cent and an average of 74.08, which is 
near the maximum of silica for granites generally. 
Pegmatite lies at the other extreme. Its mineral constituents range 
usually from one-half inch to 1 foot or even several feet in diameter. 
Tt is reported that the crystals in some pegmatite dikes measure from 
10 to 30 feet in length by 1 to :'. feet in width. The chief minerals in 
pegmatite dikes are the same as in granite, but they occur in different 
though varying proportions. With these minerals are often associated 
tourmaline, garnet, beryl, etc. Chemically these dikes generally eon- 
tain more silica than the granite. Dikes of pegmatite are, as a rule, 
more irregular in width than those of aplite. They generally range 
in thickness from 1 inch to 10 feet. 
The courses of the pegmatite dikes in the Maine quarries and their 
relation to the structural features are shown in figs. 1. 6, 20, 21, 24, 
28, 30, 31, and 33. Their courses are distributed as follows in IT) 
quarries : 
Courses of i><</in<iliic dikes. 
Numbor of quarries. 
N 3 
N. 20° E 1 
X. t0° E ,___ 1 
x. 60° e l 
x. ir, --jo w •". 
x. t5° w l 
X. 65°-70° W 2 
X. S0°-00 o E 2 
I [orizontal 1 
PI. X, /?. shows a pegmatite dike crossing the diorite (black granite) 
at Round Pond. in Lincoln County. At the Hallowell Granite Works 
(Longfellow) quarry (p. 119) a 2-foot dike consist^ of milk-white 
"Bull. IJ. S. Geol. Survey No. 148, 1897, pp. 124, L50, ^<>t;, 219. 
