32 THE GRANITES OF MAINE. 
Turner a calls attention to the sheet structure and exfoliation of 
Fairview dome in the Yosemite. 
Gilbert 6 shows that sheet structure occurs in synclinal as well as 
in anticlinal attitude— in other words, is parallel to hollows as well 
as hills— which he considers unfavorable to the theory that it is an 
original structure. He suggests that sheet structure may possibly 
be due to expansive stress consequent upon relief from compressive 
stress brought about by the removal of the mass into which the 
granite was intruded. Subordinately he notes that in the Sierras, 
at least, the dome structure and the parallel joint structure do not 
occur in the same place and that the former has resisted general 
erosion more successfully than the latter. 
Dr. G. F. Becker, in a recent conversation with the writer, stated 
that he had found the granites and gneisses at the bottom of the Colo- 
rado Canyon both vertically and horizontally jointed. If these are 
true granites and are still in contact with the rocks into which they 
were intruded and show genuine sheet structure the phenomenon 
would conclusively prove that such structure may occur indepen- 
dently of solar heat and load. 
Mr. S. F. Emmons similarly stated that in the Mosquito (Park) 
Range, in Colorado, the pre-Cambrian granite and schist are cut by 
horizontal joints to a depth of 50 feet below their contact with the 
overlying Cambrian, the joints diminishing in number downward. 
The original load upon the granite here consisted of at least 10,000 
feet of Paleozoic and between 5,000 and 6,000 feet of Cretaceous rocks. 
As the granite, however, was not intruded into Cambrian sediments 
it must have been exposed to atmospheric erosion before they were 
deposited. These horizontal joints may therefore have been related 
to solar temperature. 
Mr. G. K. Gilbert has recently studied the granite domes of 
Georgia and attributes their sheet structure to compressive strains. 
He finds that the granite in these domes c is not naturally divided into 
plates, but that the outer parts of the granite — the parts nearest 
the surface — are in a condition of compressive strain, which results 
in slow exfoliation and which enables quarrymen, b} T means of care- 
fully regulated blasts, to develop joints that run approximately 
parallel to the surface, so that the granite is detached in sheets. As 
these sheets are divided into blocks in the process of quarr}dng the 
blocks expand horizontally as they are released from the general 
mass. In these granitic domes parting planes also develop naturally 
a Turner, H. W., The Pleistocene geology of the south-central Sierra Nevada, with 
especial reference to the origin of the Yosemite Valley: Proc. California A.cad. Sci., 3d 
ser., Geology, vol. 1, No. 9, 1900; Formation of domes, pp. 312 315, and PI. XXXVII. 
6 Domes and dome structure of the high Sierras: Bull. Geol. Soc. Am., vol. 15, pp. 
29-36, and pi. 3, 190 4. 
c Letters to writer dated May 4 and June 11, 1906. 
