STRUCTURE OF MAINE GRANITES. 37 
face, closing up the channel to half its original width. The practica- 
bility of developing sheet structure by the use of explosives and com- 
pressed air, as it is developed in some of the North Carolina granite 
quarries, shows that the rock is under a compressive strain there." 
All these observations bring this theory within the domain of induct- 
ive science. If sheet structure is due to compressive strain, it is due 
to such a strain as would produce a series of undulating fractures 
extending entirely across a granite mass several miles in diameter and 
to a depth, as far as observed, of 175 feet from the rock surface. 
6. In view of the undoubted sheeting effect of expansion under 
solar heat within a short distance of the surface and of the fact that 
some of the sheets near the surface measure but a • few inches in 
thickness, it is quite possible that very thin surface sheets have 
originated in this way: but in view of what was stated under 5 it 
seems rather probable that compressive -train i- the main factor in 
producing massive sheets. At the surface both causes may have 
cooperated. The progressive thickness of the sheets downward indi- 
cates that the operation of this st rain is evidently also dependent upon 
distance either from the present surface or from a former surface or 
contact. 
According to this view sheet structure may be said to exerl a con- 
trolling influence upon surface forms, yet it seems quite admissible 
that granite domes as conspicuous a- Stone Mountain, in Georgia, and 
Fairview Dome, in California, notwithstanding all the exfoliation 
that has taken place on them or the erosion they may have suffered, 
may still retain some degree of parallelism between their present 
form and the original contour of the granitic intrusions of which 
they are parts. This may be true. also, of the granite hills of Mount 
Desert. 
The probability being admitted that the general parallelism be- 
tween the present surface and the sheet structure is the result of 
erosion that followed the sheeting, the question still remains. What 
has determined the form and location of the domes? These may 
possibly be referred to major arches (anticline-) in the folds of the 
stratified rocks which originally overlay the granite. The crusta] 
movement that produced these folds may also have brought about the 
intrusion of the material that formed the domes beneath them. 
Although the sheet structure and the rock surface are very generally 
parallel, they are not universally so. as may be seen on the west 
flank of Mosquito Mountain, shown in PL III, Z>, which has evi- 
dently been partially eroded, and at the Clark Island (p. 126) and 
Sprucehead (p. 12-I-) quarries, where the rock surface and the sheet 
structure were also found to be discordant. 
" Watson, T. L., and Laney, F. B., The building and ornamental stones of North 
Carolina : Bull. North Carolina Geol. Survey No. 2, 1906, pp. 157-160. 
