36 THE GRANITES OF MAINE. 
These propositions will be considered in the order given : 
1. Solar heat may produce a certain amount of exfoliation in thin 
sheets at the surface, as is proved experimentally in the fire method 
of granite quarrying in India (p. 33), but as it penetrates only to a 
depth of 40 feet and as sheet structure is known to occur on Crotch 
I. -land, Maine, at a depth of 140 feet and at Quincy, Mass., at a 
depth of 175 feet, it is quite inadequate to account for sheets that 
are 20 to 30 feet thick and 100 to 175 feet below the surface. 
2. In view of the load under which granite was probably formed, 
as shown by the well-known calculations of Sorby and AVard," and 
the very gradual rate at which, therefore, it probably cooled, which 
is also indicated by the general coarseness of its texture, it is very! 
improbable that the temperature at its contact surface and the tem- 
perature at depths 100 or 200 feet below could have so greatly dif- 
fered as to bring about such a system of joints by contraction. 
3. As Gilbert slates, in suggesting the theory of fracture by relief 
of tensile strain through the erosion of overlying masses, we have no 
distinct knowledge of it. It is a possible explanation. 
4. Careful inspection of the rock on both sides of the sheet joints 
fails to show any difference in texture or mineral composition. The 
sheet structure traverses both rift and flow structure, and it would be 
possible to procure specimens showing a sheet joint traversing a single 
crystal of feldspar. Whatever chemical action has taken place along 
the sheet joints is of secondary character. Acid waters may have 
gained access to the joint, but have not caused it. (See matter under 
heading Discoloration, w * Sap." etc, p. 52.) 
5. The condition of strain described by Merrill and Gilbert as exist- 
ing in the granite domes of Georgia and by Niles an<l Emerson in the 
gneiss at Monson, Mass./' and occurring to a lesser extent in some 
Maine quarries (sec pp. 93, L12, 121, 155), shows that granite and 
gneiss are in places still under compressive strain. Another instance 
occurs at the quarry of the New England Granite Works, at Concord] 
N. PL, recently visited by the writer. The foreman at this quarry was 
in the habit of calling certain sheets, marked by the absence of dis- 
coloration, "strain sheets," to distinguish them from the others. Atj 
one place a northwest-southeast compressive strain had actually ex- 
tended the strain sheet about 5 feet, and also caused a vertical frac- 
ture that extended over 15 feet diagonally from the north-south work- 
ing face to a point on a vertical east-west channel 5 feet back of the; 
° Sorby, II. ('., On the microscopic structure of crystals, indicating the origiu of min- 
erals and rocks: Quart. Jour. Geol. Sue. London, vol. 14, 1858, pp. 453 el seq. ; Ward, 
.1. Clifton, On the granitic, granitoid, and associated metamorphic rocks of the Lake dis- 
trict : Ibid., vol. 31, 1st:., pp. 568-602. 
6 Niles, W. II., Some interesting phenomena observed in quarrying: I'roc. Boston Soc. 
Nat. Hist, vol. it. 1872, pp. so ST. and vol. 16, 1874, pp. 41-43. Emerson, B. K., 
Geology or old Hampshire County, Mass.: Mon. U. S. Geol. Survey, vol. : , . , .», 1898 
pp. G3-Go. 
