THE MECHANICAL INTERPRETATION OF JOINTS 25 
further. For with only one of the two intersecting planes of 
shearing given, we cannot even guess at the position of the principal 
stresses.* 
3. The New England region is that part of the United States of 
which we know most definitely and quantitatively that it has under- 
gone considerable deformation in post-glacial time. <A glance at 
Fairchild’s map of ‘“‘Isobases of Post-glacial Uplift”? shows that 
if we can assume the earth’s surface to the south and southeast 
to be relatively at rest and the domelike upheaval with its center 
halfway between Quebec and James Bay to be rising independ- 
ently, there should exist a compressive stress in the general 
direction northwest-southeast throughout the surface of New 
England. It may, of course, be mere coincidence that this is the 
same direction which we found required to produce the ‘‘double- 
sheet”? structure on Robeson Mountain. But, combined, these 
three observations are distinctly favorable to the view that the 
larger part of the remarkable sheet structure of the granites of 
New England is due to compressive stresses arising from the larger 
earth movements indicated by Fairchild’s isobasic maps. 
The objection may be raised that in quarries in other parts of 
New England strains in different directions have been observed. 
Great variations in local conditions of stress are not surprising 
in view of two important facts. The direction of relief in these 
cases is normal to the surface. Where the surface is far from 
horizontal the position of one of the two principal stresses must 
vary from place to place, and with it, the position of the resulting 
plains of shearing. 
Furthermore, observation shows that the granites, which 
exhibit the sheet structure, are divided by various joint systems, 
some unquestionably of earlier origin. It is, therefore, not an 
unbroken sheet of granite that is being deformed in this region, but 
a mosaic of large and small blocks. ‘The stress conditions, resulting 
t Tf, for instance, in the case illustrated in Fletcher Quarry only the planes striking 
N. 20 W. had been given, we would probably not have considered: them to be in 
harmony with a compressive stress acting from the northwest. 
2H. L. Fairchild, ‘‘ Post-glacial Uplift of Northeastern America,” Bull. Geol. Soc. 
Amer., Vol: XXIX (1918), p. 202. 
