38 THE GRANITES OF MAINE. 
Sheet structure in granite so much resembles the structure of 
folded stratified rocks thai underground water circulates in prac- 
tically the same way along the fracture planes of one and bedding 
planes of the other. The exudation of water along sheet joints on 
vertical rock faces is seen in many of the Maine quarries, and is 
shown in PI. VI,#. 
That sheet structure is not confined to intrusives is shown at 
quarries in NTiantic, R. I., and Milford, N. H., where it passes indif- 
ferently from the granite into the overlying gneiss. 
I IeiTinann " divides joints into two groups — joints formed by lateral 
compression, whose distances from one another are related to the 
coarseness of the rock texture, and joints due to expansion, some of 
which are parted and filled with calcite, quartz, pegmatite, or vol- 
canic rock. That many joints are due to compressive or torsional 
strain, and that every such strain resolves itself into two components, 
resulting in two sets of joints that intersect at an angle of about 90°, 
each forming an angle of about 45° with the direction of the strain, 
are facts now generally recognized. Crosby & has suggested that 
torsional strains may have been supplemented by vibratory ones in 
causing joints. Becker, c in a recent paper, shows that four or even 
more than four systems of joints may be due to a single force, lie. 
also shows that subsequent strain on a region thus jointed would 
tend to produce motion along the previous^ formed joints rather 
than a new system of jointing. It is conceivable that if a region had 
been jointed and afterwards subjected to a tensile strain, some of its 
joints might be parted, and if they were very deep the openings 
might become filled with volcanic matter from below, or, if not, 
with matter from above, infiltrated from overlying rocks. That 
motion has occurred along some of the joints in the Maine quarries 
is evident from the polished and striated surfaces of the joints as 
well as from .the faulting of the sheets. 
The structural diagrams in Part II, accompanying descriptions of 
the quarries, show the course and dip of the joints at the Maine 
quarries. The intersection of sheet structure bv joint structure is 
shown in Pis. IV, A, B; V, B; VI, A, B; VIII, A; IX, .1. The 
conspicuous east-west system of joints as seen on Crotch Island is 
shown in PL II, B, and as seen in the region of Vinalhaven, on 
Heron Neck, at the south end of Green Island, in PL II. .1. 
op. cit., ik 103. 
b Crosby, W. ()., The origin of parallel and intersecting joints: Am. Geologist, vol. 12, 
1893, pp. 368 375. 
c Becker, George I\. Simultaneous joints: Proc. Washington Acad. Sci., vol. 7, July, 
1905, pp. L'CT-L'Tr,, PI. XIII. 
