ROCK VARIATIONS. 45 
soda-lime and potash feldspars (oligoclase and microcline), smokv 
quartz, biotitc, and muscovite (black and white mica), and garnet. 
The feldspars, quartz, and micas attain a length of several inches. 
At the North Jay quarry (p. 82) the pegmatite dikes measure up to 
2 feet G inches in width and consist of a milk-white potash feldspar, 
smoky quartz, biotite muscovite, the constituents measuring several 
inches in diameter. At the Clark Island (J. C. Rodgers) quarry 
(p. 126) there are two intersecting pegmatite dikes with similar 
material of similar dimensions, together with black tourmaline and 
garnet. The granite at Fryeburg, near the New Hampshire line, 
abounds in pegmatite. At the Eagle gray granite quarry (p. 1-J4 and 
fig. 31) two dikes, one 5 feet, the other 10 feet thick, alternate with 
granite 25 and GO feet thick. The feldspar masses and crystals 
attain a length of 12 inches and the biotite and muscovite crystals 
and the quartz masses a length of 6 inches. Small garnets are 
abundant. Mingled with the pegmatite is some fine-grained aplitic 
material. There is also considerable pegmatite at the Waldoboro 
quarry, in Lincoln County. (See p. 142 and fig. 29.) At the Wild 
Cat or Willard Point quarry of the Bodwell Granite Company, now 
abandoned, there is a 12-inch pegmatite dike of feldspar, quart/, 
muscovite, and black tourmaline, which has a banded structure. It 
is crossed by another dike half as thick, with a difference in strike 
of 20°. 
The origin of pegmatite has been much discussed both in Europe 
and in this country." The coarseness of its constituent minerals indi- 
cates slow crystallization, and the irregularity of the dikes shows 
tensional rather than torsional strain. The banding of some peg- 
matite dikes and the isolated lenticular character of others indicate 
that the dikes were filled from heated solution, while many of them 
differ in no respect from dikes of igneous origin except by the coarse- 
ness of their texture. For these reasons it is thought that pegmatite 
dikes in granite have been formed in openings and fissures that were 
due, possibly, to contraction while the granite was still hot and that 
some of these openings were filled with matter in a state of both 
molten plasticity and solution under pressure, and others by heated 
solutions that gathered matter from the adjacent granite. Howso- 
ever derived, this dike material crystallized very slowly. 
Granite. — Finally, there are dikes that differ from all those just 
described, formed simply of fine or medium-grained granite. Thus 
a The principal American writings on the subject are: Williams, G. II., The general 
relation of the granitic rocks in the middle Atlantic Piedmont Plateau ; Fifteenth Ann. 
Rept. U. S. Geol. Survey. 1895, pp. 675-684; Crosby, W. O., and Fuller, M. L., Origin of 
pegmatite: Techn. Quarterly, vol. 9, 189G, pp. :;26-:jr>6 ; Am. Geologist, vol. 19, 1897, 
pp. 147-1S0; Van Ilise, C. R., A treatise on metamorphism : Mon. U. S. Geol. Survey, 
vol. 47, 1904, pp. 720-728. 
