STRUCTURE OF MAINE GRANITES. 33 
within a few inches of the surface, and the expansive force is there 
so great as to induce conspicuous buckling- in the thin sheets thus 
formed. This buckling is illustrated in PL VII, .1, from a photo- 
graph taken by Mr. Gilbert on Rock Chapel Hill, near Lithonia. 
The jar of blasting precipitates this sheeting action, so that several 
of the domes at which quarrying is in progress show long -lines of 
freshly formed disrupted arches. Mr. Gilbert found that the hori- 
zontal elongation, or rather the elongation coincident approximately 
with the contour of the dome surface, amounted, by one measurement, 
to three-fourths inch in a length of 40 feet. 
The artificial production of sheets in granite, as practiced at 
Bangalore, in southern India, shows similar phenomena. It v is 
described by H. Warth a in substance as follows : At the surface there 
is a horizontal sheet of rather weathered rock 4 feet thick ; below this 
lies a sheet of fresh rock 3 feet thick, but below this lies fresh rock 
without split. These sheets " are probably due to the variations of 
temperature, daily and seasonal." By means of wood fires plates 
60 by 40 feet by 6 inches in thickness are detached in one piece. A 
line of fire 7 feet long is gradually elongated and moved over the 
granite. The effect of the fire is tested by hammering the granite 
in front of it, and then the fire is moved forward. The maximum 
length of the arc of fire is 25 feet. The burning lasts eight hours; 
the line of fire is advanced 6 feet per hour. The area passed over 
by line of fire is 460 square feet. The amount of wood used is 15 
hundredweight. The average thickness of stone is 5 inches and its 
specific gravity is 2.62. These data show that 30 pounds of stone 
are quarried with 1 pound of wood. Some plates are taken out 
in inclined position. The action of fire is independent of the orig- 
inal surface of rock and also of the direction of lamination (the 
granite is gneissose) and of veins. The uniformity in the thickness 
^f the sheets is attributed to the regulating influence of preexisting 
racks. 
Van Hise, in his treatise on metamorphism, 6 is inclined to attribute 
heet structure to solar temperature. 
Before these various views are discussed the sheet structure as 
xposed at the Maine quarries will be described. 
Dome form and sheet structure are most finely exhibited at Crotch 
[sland, near Stonington, and at Mosquito Mountain, near Frank- 
ort. PL II, Z?, shows the structure in the southern half of Crotch 
[sland, at Thurlow Head. The dome is oblate, measuring about 
1,500 feet from north to south and 140 feet in height. 
" The quarrying of granite in India : Nature, vol. 51, 1895, p. 272. 
6 Mon. U. S. Geol. Survey, vol. 47, pp. 434-439. 
3495— Bull. 313—07 3 
