STRUCTURE OF MAINE GRANITES. 35 
faulting of the sheets is likely to occur also along the steep joints. 
[See also p. 98.) PL IX, A, a view taken at the Waldoboro quarry, 
shows the relation of the sheet structure to the underside of the 
)riginally overlying mass of schist, a remnant of which bounds the 
marry on two sides at the top. The sheets here are nearly horizon- 
tal, while the schist dips 45°. 
The observations as to sheet structure made at over 100 granite 
quarries in Maine are here summarized : 
1. There is a general parallelism between the sheets and the rock 
surface, resulting in a wavelike joint structure and surface over 
arge areas. 
2. The sheets increase in thickness more or less gradually down- 
ward. In the coarse-textured granites of Crotch and Hurricane 
slands the increase is abrupt. (See Pis. Ill, A, and IV, .4.) 
3. The sheets are generally lenses, though in some places their 
! orm is obscure. Their thick and thin parts alternate vertically with 
me another. The joints that separate these superposed lenses there- 
! ore undulate in such a way that only every other set is parallel. 
4. On Crotch Island the sheet structure extends to a depth of at 
east 140 feet from the surface. 
5. There are indications here and there that the granite is under 
:ompressive strain, which tends to form vertical fissures or to expand 
he sheets horizontally so as to fill up small artificial fissures. (See 
:>. 34.) 
The observations made in Europe and in this country, taken in 
connection with the various inferences geologists have drawn from 
hem, indicate that sheet or " onion " structure in granite rocks is 
lue to the following possible causes: 
1. To expansion caused by solar heat after the exposure of the 
granite by erosion. 
2. To contraction in the cooling of the granite while it was still 
mder its load of sedimentary beds, the sheets being therefore ap- 
proximately parallel to the original contact surface of the intrusive. 
3. To expansive stress or tensile strain brought about by the dimi- 
mtion of the compressive stress in consequence of the removal of 
lie overlying material. 
4. To concentric weathering due to original texture or mineral 
composition. This action would be chiefly chemical and would be 
dded by vertical joints and by any superficial cracks due to expan- 
sion and contraction under changes of temperature. 
5. To compressive strain akin to that which has operated in the 
! olding of sedimentary beds. 
6. To the cause named under 1 at the surface, but to the cause 
lamed under 5 lower down. 
