6l2 STUDIES FOR STUDENTS 



Crosby has recently suggested' that when torsion has nearly, 

 but not quite, reached the limit of elasticity in rocks, an earth- 

 quake shock may act as the trigger which sets the process in 

 motion, and he thus combines this cause with torsion in explain- 

 ing joints. Crosby also emphasized the fact that a plate when 

 subjected to torsion does not crack along a single plane of frac- 

 ture, the weakest plane, but that many parallel fractures are pro- 

 duced. He applies this fact to rock beds, and suggests that the 

 fracture must first occur at some one plane, that of the greatest 

 stress or least strength in the distorted belt. The shock of the 

 fracture, added to the force of torsion, goes beyond the ultimate 

 strength of the beds at the next weakest plane, thus fracturing 

 them and producing another joint, and so on, until the compli- 

 cated fracturing actually obtained in the glass plate is paralleled 

 by the rocks. He thus makes the rupture of the first joint itself 

 serve the purpose of a secondary shock, and this rupture a third 

 shock, and so on, producing a set of joints in rapid succession 

 over an" extended area. Finally a place is reached where the 

 rock has not been sufficiently distorted for the shock of the last 

 fracture to carry the stress beyond the breaking strength. This 

 theory is as applicable to simple tension as to torsion. In the 

 above it appears that Crosby has overlooked the fact that he has 

 not explained the first earthquake shock. The statement might 

 be amended as follows : The first fracture occurs because the 

 steadily acting orogenic forces finally go beyond the ultimate 

 strength of the deformed beds. When rupture takes place, this 

 gives the first shock. This initial shock carries the stress beyond 

 the ultimate strength of the next weakest planes, and so on. 



Compressio7i joints. — Daubree^ and Becker 3 show that joints 

 may be produced by compression. In this case there will be 

 jomting in two planes when the rocks are simply folded, and, 



'The origin of parallel and intersecting joints, by W. O. Crosby, Am. GeoL, 

 Vol. XII, 1893, pp. 368-375. 



^Geologic experimentale, par A. Daubree, pp. 315-322, Paris, 1879. 



3 George F. Becker : Finite homogeneous strain, flow, and rupture of rocks. 

 Bull. Geol. Soc. Am., Vol. IV, pp. 41-75, 1893. The torsional theory of joints. 

 Trans. Am. Inst. Min. Engineers, ol. XXIV, pp. 130-138, 1894. 



