416 ' ROLLIN T. CHAMBERLIN 
PART II. THE INTIMATIONS OF SHELL DEFORMATION 
As the result of field studies in Pennsylvania in 1905, the 
crustal shortening involved in the folding of the Appalachian 
Mountains west of Harrisburg was found to have been fifteen 
miles. Computations based upon this shortening and the con- ~ 
sequent upbowing seemed to indicate that the shape of the deformed 
section was that of a triangular prism or wedge pointed down- 
ward.t Two sides of the wedge were found to converge beneath 
the mountainous tract till they came together under the middle 
portion of the deformed belt at a depth of thirty-two miles. No 
consideration whatever of stress-and-strain relations entered into 
the deduction of this wedge-shaped form. It came out directly 
from the graphic treatment of the field measurements, quite 
irrespective of any theory of mechanics, or of the nature of diastro- 
phism; in fact, the result came as a distinct surprise. It was 
soon seen, however, that the shape was in harmony with the 
principles of fracture under essentially uniform horizontal com- 
pression. 
The key to the method used in this inquiry lay in the axiomatic 
proposition that there must be a definite relation between the 
thickness of the deformed shell, the horizontal shortening which 
this shell has suffered, and the amount of resulting vertical bulge, 
on the assumption that there has been no notable compacting of 
materials.2 When the horizontal shortening and the consequent 
upswelling have been determined from field measurements, the 
thickness of the deformed shell can readily be calculated. The 
most important outcome of the method is that it gives the under- 
configuration of the deformed shell in addition to other qualities. 
It is from this feature that the most important intimations relative 
to the deeper diastrophism are drawn. 
Besides the under-configuration of the deformed shell, the 
following generalizations seem to be warranted: (a) Sharp folding 
‘Rollin T. Chamberlin, ‘‘The Appalachian Folds of Central Pennsylvania,’ 
Jour. Geol., Vol. XVIII (1910), pp. 228-51. 
2 For a discussion of this and other possible qualifying factors, see Jour. Geol., 
Vol. XVIII (1910), pp. 236-37, and also ‘‘The Building of the Colorado Rockies,” 
Jour. Geol., Vol. XX VII (19019), pp. 235-38, 244-47. 
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