. Transactions. and Vice Versa. In an ideal material, as before stated, the fractures producedby compression will intersect the axis of pressure at an angleof 45°. Suppose that in such an ideal material two sets offractures showing conjugate relations are found, but that differ-ential motion has not taken place or cannot be determined byinspection of the fractured surfaces. It is evident that obser-vation will not lead to establishing the direction in which therupturing force was applied because there are two possible VEIN-FILLED OPENINGS IN SOUTHEASTERN ALASKA. 583 axes from which to choose.
. Transactions. and Vice Versa. In an ideal material, as before stated, the fractures producedby compression will intersect the axis of pressure at an angleof 45°. Suppose that in such an ideal material two sets offractures showing conjugate relations are found, but that differ-ential motion has not taken place or cannot be determined byinspection of the fractured surfaces. It is evident that obser-vation will not lead to establishing the direction in which therupturing force was applied because there are two possible VEIN-FILLED OPENINGS IN SOUTHEASTERN ALASKA. 583 axes from which to choose. Thus, in Fig. 3, the compressioncausing rupture on aa and afaf may have been applied alongHH or along VV. Considering fractures in nature, the foregoing deductionsmay be applied in a general way without taking up the modi-fication in the angles between fractures, due to non-homoge-. Fig. 2.—Fracture in a Square Prism Restrained on Two Opposite Sides,B, but Free to Deform on the Remaining Side, A. Result : TwoSets of Fractures in Conjugate Relation. neity and lack of perfect elasticity in the rocks, or to prelim-inary deformation by cubic compression. For a case of comparative simplicity, it may be imaginedthat a block of the lithosphere is moved out of its place in adirection opposed to gravity, or, what amounts to the samething, the surrounding rocks may be supposed to have beendrawn away from any block. By such an adjustment ofmasses, equilibrium in the blocks is disturbed, and, because the 584 VEIN-FILLED OPENINGS IN SOUTHEASTERN ALASKA. restraint of the neighboring masses has been removed, therewill be a tendency for the block, which is differentially raised,to flatten out under the action of gravity. In deformationunder these conditions the weight of the block takes the placeof an outside force which is commonly called upon in account-ing for fractures obs
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