ROCK VARIATIONS. 49 
opened previously formed joints or made new ones deep enough to 
be injected with volcanic material. How far this may have pene- 
trated the rocks which overlay the granite or whether it overflowed 
at their surface can not even be conjectured. 
At the granite quarries, wherever this course is possible, the dikes 
and the headings are left to form the bounding walls of the exca- 
vations. 
SEGREGATIONS (KNOTS). 
Quarrymen know too well that granite is often disfigured by gray 
or black " knots " of circular or oval irregular curved outline, rang- 
ing in diameter from half an inch to 3 feet and exceptionally even 10 
feet. These were studied by geologists long ago. a They are finer 
grained than the granite in which they occur, contain nearly 10 per 
cent less silica, much more black mica or hornblende (which accounts 
for their darkness), and a little more soda-lime feldspar, and their 
specific gravity is about 0.09 per cent higher. 
PI. V, A, shows 12 knots in the vertical cuts at Crabtree & Havey's 
quarry, in Sullivan. As the strength and durability of the stone 
are in no wise affected by the " knots," the blocks containing them 
are used for curbing, crossings, or other constructions where color 
and shade are not taken into consideraton. 
A thin section of a very dark gray knot from this quarry shows 
a much greater abundance of biotite than the granite. The feldspar 
and the biotite particles in the knot measure up to 0.5 mm. (one- 
fiftieth inch), whereas in the granite the feldspar measures up to 
2.25 mm. and the biotite up to 0.75 mm. 
A knot from the Palmer quarry, on Vinalhaven, is of medium- 
gray shade, with a very fine grained groundmass inclosing porphy- 
*itic buff-pinkish feldspars and smoky quartz particles up to about 
ne-fourth inch in diameter. The groundmass consists of quartz, 
botash feldspar (microcline), soda-lime feldspar, black mica, and 
lornblende in particles ranging in size from 0.0T5 to 0.5 mm. The 
3orphyritic particles are of quartz, potash feldspar, or soda-lime feld- 
par, and hornblende, and measure from 0.75 mm. up. 
A very dark, almost black, knot from the Sands quarry, on Vinal- 
laven, consists of crowded particles of hornblende and biotite, which 
ompose one-half the knot, the rest being mostly soda-lime feldspar 
ind quartz. Among the knots noticed is a spherical one, 2^ feet in 
" Phillips. J. A., On concretionary patches and fragments of other rocks contained in 
[ranite: Quart. Jour. Oeol. Hoc. London, vol. 36, 1880, pp. 1-22. PI. I. Merrill, C P.. 
n the black nodules or so-called inclusions in the Maine granite: Proc. U. S. Nat. Mus., 
ol. 6, 1883, pp. 137-141. Grimsley, G. P., P.asic segregations: The granites of Cecil 
ounty in northeastern Maryland: Jour. Am. Soc. Nat. Hist, Apr. and July. 1804. Daly, 
eginald A., Basic segregations : The geology of Ascutney Mountain, Vermont : Bull. I\ S 
eol. Survey No. 209, 1003, p. KM. 
3495— Bull. 313—07- 
