STRUCTURE OF MAINE GRANITES. 41 
the sheet, through a maximum thickness of inches. It is coex- 
tensive with the discoloration known as " sap " and occurs at many 
places near vertical joints. Under the microscope this structure 
proves to consist of minute, nearly parallel fissures, of no great con- 
tinuity, which traverse the mineral particles and which in the thin 
section examined are especially conspicuous in the quartz and the 
mica. The distance between these fissures measures from a tenth to 
a half a millimeter, or from one two hundred and fiftieth to one-fif- 
tieth inch. The parallelism both to the sheets and the " sap " and its 
relation to the vertical joints indicates that the structure may be due 
to the freezing of surface water which has found its way to the sheets 
through the vertical joints and has entered the rift fissures. 
The writer^ attention was called to a similar structure in a quarry 
at Milford, N. H., consisting of short, parallel fractures along the 
rift, from one-half inch to 2 inches apart, having no apparent con- 
nection with joints or discoloration. This is probably due to strain 
affecting part of the granite mass. 
SUBJOINTS. 
Careful inspection shows that the joint structure in the Maine 
granites does not everywhere consist of a simple fracture, but that 
it is at many places complex. Minute fractures branch off from the 
joint at an acute or right angle and penetrate the rock a few inches, 
or the rock for a few inches on either side of the joint is traversed 
by microscopic fissures that are roughly parallel to it. All such 
structural features may properly be called subjoints. 
A thin section of North Jay granite across a joint face shows two 
diverging subjoints that form an acute angle with each other and 
with the main joint and are filled with limonite and sericite (?). 
Single subjoints are, however, rarely found, five or six fine parallel 
fissures generally occurring together. In one of the quarries at 
Franklin (quarry of W. B. Blaisdell & Co., p. 91), the subjoints 
are parallel to the main joint, and as both main and subjoints are 
filled with calcite, the granite near the joint weathers out vertically 
in small slablike pieces from one-half inch to 2 inches thick, con- 
sisting of a central band of calcite, with one of granite on either 
side. Under the microscope one of these subjoints, measuring 0.74 
mm. across, is seen to be filled with long slivers of quartz and feld- 
spar and scales of biotite, forming a breccia. Another, 0.07 mm. 
wide, is filled with secondary quartz. At the T. M. Blaisdell quarry 
in East Franklin. Hancock County (p. 93), a northeast-southwest 
vertical joint has on one side numerous subjoints that meander off 
at right angles to it and traverse a cubical mass whose sides measure 
10 to 15 feet. At the Shattuck Mountain quarry, in Calais (p. 1G4), 
