208 O. F. Becker — Im/pact Friction and Faulting. 



new proofs are given above and the anatysis is further extended. 

 That a fault under a compressive stress in a mass of rock cut 

 by a single fissure will tend to produce a system of parallel 

 fissures, I showed in my former paper ; I have nothing to add 

 on that subject. This fact explains the formation of parallel 

 veins, the parallel fissure of step-faults and the sheeted structure 

 of landslides which are merely superficial faults. 



Faults are known to be in a large proportion of cases the re- 

 sult of a tendency to fold carried bej^ond the limit of elasticity 

 of the rock. The simplest case of faulting is therefore an anti- 

 clinal fault passing over into a fold at its extremities. I have 

 shown that under certain conditions instead of a single fissure 

 a system of parallel fissures will be formed, and that the move- 

 ment will be distributed over all of them. The upper end of 

 any sheet will then remain parallel to its original position. In 

 a stratified country intersected by such a fissure system the re- 

 sult is, strictly speaking, a system of monoclinal faults. The 

 divisions of the step-faults which I have personally inspected, 

 however, are all too insignificant in thickness to be ordinarily 

 or conveniently classified as monoclinal faults. That this is 

 not always the case is apparent from Mr. Greikie's illustration* 

 of a step-fault in the coal fields between Linlithgow loch and 

 the Frith of Forth. The logarithmic character of this fault is 

 manifest from the illustration, while, if I understand the geog- 

 raphy aright, the separate benches must be as much as a quarter 

 of a mile in width. Even if the section is not to scale, the 

 width of the sheets must evidently be such that no one would 

 hesitate to include the case among monoclinal faults. In this 

 particular instance the strata are practically horizontal. This 

 is a matter of no consequence however, for, had they been in- 

 clined previous to the intersection by the fissure system, they 

 would still be inclined at the same angle. Thus it is certain 

 that some monoclinal faults are explicable on the theories set 

 forth above and their relation to anticlinal faults is at once evi- 

 dent. They result from the substitution of a system of fissures 

 for a single fissure in an anticlinal fold followed by the distribu- 

 tion of motion over all the contacts. In all such cases it is un- 

 necessary to suppose a tilting of the monoclinal blocks in a 

 plastic magma after separation from the adjoining country, or 

 to conclude that the fissures have gaped to allow of such tilt- 

 ing ; for step-faults involve the compressive strains which most 

 geological phenomena lead us to suppose the rule in geological 

 dynamics. A series of blocks tilted after separation from the 

 surrounding country also leave a triangular space at each end 

 of thesystem which must apparently fill with the plastic magma, 

 a complication which will not be met with in step-faults. This 



* Textbook of Geology, p. 532. 



