Fig. 413.
Fig. 414.
Fig. 412.—A possible interpretation of [Fig. 411]. (Dana.)
Fig. 413.—A possible interpretation of [Fig. 411]. (Dana.)
Fig. 414.—A possible interpretation of [Fig. 411]. (Dana.) ]
Fig. 415.—Jointed rocks. Cayuga Lake, N. Y. (Hall.)
Joints have been referred to various causes, among which tension, torsion,[214] earthquakes,[215] and shearing[216] are the most important. Most of them may probably be referred to the tension or compression developed during crustal movements.[217] In the formation of a simple fold, for example, tension-joints parallel with the fold will be developed, if tension goes beyond the limit of elasticity of the rock involved. If the axis of a fold is not horizontal, that is, if it “plunges,” as it commonly does, a second set of joints roughly perpendicular to the first will be developed. If the uplift be dome-shaped and sufficient to develop joints, they will radiate from the center. It is true that joints affect regions where the rocks have not been folded, and where they have been deformed but little, but deformation to some extent is well-nigh universal.
Fig. 416.—Jointing in granite. The surface of the rock is a joint plane. Northwest boundary of the United States. The edges of other joint planes normal to the surface are also shown. (Ransome, U. S. Geol. Surv.)
