512 



GEOLOGY. 



torsion/ earthquakes/ and shearing^ are the most important. Most of 

 them may probably be referred to the tension or compression developed 

 during crust al movements.^ In the formation of a simple fold, for 

 example, tension -joints parallel with the fold will be developed, if ten- 



FiG. 416. — Jointing in granite. The surface of the rock is a joint plane. Northwest 

 boundar}^ of the United States. The edges of other joint planes normal to the 

 surface are also shown. (Ransome, U. S. Geol. Surv.) 



sion goes beyond the hmit 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 upHft 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. 



' Daubree. Geologie d'Experimentale, pp. 306-372. 



2 Crosby. American Geologist, Vol. XII, 1893, pp. 368-375. 



3 Becker. Bull. U. S. Geol. Surv., Vol. X, pp. 41-75. 



* Van Hise. Principles of North American Pre-Cambrian Geology. 16th Ann, 

 Rept. U. S. Geol. Surv., Pt. I, pp. 668-672. 



