A FRACTURE VALLEY SYSTEM 95 



phenomenon, fracturing; one, a result of small or imperceptible 

 displacement; the other, of profound, or at least notable, disloca- 

 tion. The torsional experiments of Daubree produced more or 

 less rectangular systems of nearly parallel fractures, some of 

 which might be called joints, and others faults. The studies of 

 Becker,^ Van Hise,^ and Hoskins^ have established the laws and 

 frequency of parallel and of conjugate joints {systemes conjiigues 

 of Daubree) and the phenomena have become familiar to all 

 geologists who have worked in regions of well exposed disturbed 

 rocks. 



The phenomena are in part as follows : There are faults in 

 more or less parallel lines through wide extents of territory inter- 

 sected by faults at various angles often nearly 90°. 



A dominant or major fault is frequently accompanied by par- 

 allel fractures of minor degree, which are in some cases close to the 

 dominant fault, in other cases at considerable distances from it. 



Faulting is accompanied in parts of its course by crushing 

 and brecciation of the wall rocks ; in other parts the sides of the 

 fault are closely pressed together without evidences of breccia- 

 tion. Further, there are portions of a fissure plane where the walls 

 are not closely compressed, but may be actually open. 



Planes of fracture and fissuring are often lines of special 

 decomposition of the rock material, and are for this reason less 

 resistant than the neighboring rocks. 



From these facts it is evident that the course of drainage, in 

 following lines of least resistance, may cut its way along fault 

 lines where they are in brecciated, loosely aggregated, or decom- 

 posed rocks, but may leave the faults in places where unbroken 

 rocks are closely compressed. Moreover, streams may find less 

 obstacle in cutting unfractured softer rock than in removing 

 fractured harder material. 



• Geo. F. Becker, Monograph III, U. S. Geological Survey (Washington 

 1882), pp. 156-87; Bulletin of the Geological Society of America, Vol. IV (1893), pp. 

 41-75; and Transactions of the American histitute of Mtni?ig Engineers, Vol. XXIV 

 (1894), pp. 130-38. 



^C. R. Van Hise, Sixteenth A>inual Report, U. S. Geological Su7-vey (Wash- 

 ington, 1896), pp, 633-78. 



3L. M. HosKiNS, ibid., pp. 845-74. 



