408 



GEOLOGY 



partly or wholly, by detritus from above, or by mineral matter 

 deposited from solution (veins) . Many rich ore- veins are developed 

 along joint-planes. 



Joints have been referred to various causes, among which 

 tension, torsion, 1 earthquakes, 2 and shearing 3 are the most im- 

 portant. Most of them may probably be referred to the tension or 



Fig. 329. A surface of sandstone marked by numerous joints, chiefly in two 

 rectangular sets. Near Kinghorn, Fife. (H. M. Geol. Surv.) 



compression connected with crustal movements. 4 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, 



1 Daubree. Geologie d 'Experimental, Tome 1, pp. 306-37'J. 



2 Crosby. American Geologist, Vol. XII, 1893, pp. 368-37."). 



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



4 Van Hise. Principles of North American Pre-Cambrian Geology; 16th 

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



