ROCK VARIATIONS. 47 
Analysis of yellow foliated mineral specimen of quartz marled " D. XXT 7 /, 
105a, '05:' 
Si0 3 (silica) 53.28 
A1,.0 3 (alumina) 23. OG 
Fe 2 3 (ferric oxide) 0.10 
FeO (ferrous oxide) 4.30 
MgO (magnesia) 4.09 
Na 2 (soda) • 0. 65 
Iv.O (potash) r 8.90 
H.,0 (water) 0.00 
The mineral is secondary mica, probably derived from feldspar (although this 
is merely a conjecture), and will approximate sericite in composition. It occurs 
in fine scales, occasionally compacted and then resembling serpentine. Luster, 
pearly; color, greenish yellow; hardness, 2.5; specific gravity, 2.79 at 20° C. 
It is associated in the vein with quartz, pyrite, purple fluorite, and another 
mineral which has a greasy luster and contains magnesia, but which it was 
impossible to separate in a state of sufficient purity for analysis. This last 
mineral I believe to be talc. 
DIKES, BASIC 
Dikes of dark -greenish or black, hard and dense rock (diabase, 
rarely basalt) are of very common occurrence in the Maine quarries. 
The courses of some of these dikes and their relation to the joints are 
shown in figs. 9, 10, 14, 31, 34, 36, 38, and 39. The courses of 23 dikes 
are distributed as follows: 
Courses of 23 haste dikes at Maine granite quarries. 
N. 10°-12° E 3 
N. 22°-30° E 5 
N. 40° -50° E 6 
N. 70° E 1 
E.-W 2 
N. I5°-30° W 4 
N. J5° W 1 
N. 75° W 1 
The northeast, north-northeast, and north-northwest courses are thus 
the most common. The dikes are vertical, or nearly so, and range 
in width from 1 inch to 7 feet or more, cutting the granite sheets 
with mathematical definiteness. PL VIII, B, shows one of these 
dikes on Mount Desert, which has a course N. 15° W., and has been 
faulted from east to west, or west to east, along a gently inclined 
sheet with a displacement of 16 inches. A few feet beyond this point 
the same dike has been faulted along a northeast-southwest vertical 
joint with a displacement of 5 feet. Although it might seem that 
this dike w T as injected into the granite before the sheet structure 
w T as formed, it is quite possible that the sheet structure preceded the 
dike and that at a later time faulting affected both the sheets and the 
dike, cracking the dike along the sheets when it did not actually 
fault it. 
