Vol. 65.] OF THE CALIFORNIA^ EARTHQUAKE OF 1906. 17 



the great majority of lode-displacements — in Cornwall for instance — 

 could be accounted for by a simple ' descent of the hanging wall ' 

 had led many students of fault-phenomena in recent years some- 

 what to underrate the importance of the small minority of cases 

 where lateral and even rotational movements were postulated. 

 This observed fact in California was useful in restoring to some 

 extent the old and broader idea of fault-movement. 



Dr. J. W. Evans thought that, although the permanent deforma- 

 tion, to which the Author gave the name mochleusis, and the 

 faulting were no doubt due to a common cause which remained to 

 be determined, yet the vibratory movement or orchesis (as the 

 Author termed it) must be the direct result of the sudden snap or 

 fracture of the strata in faulting. 



Mr. G. Barrow, referring to the lateral displacement along a 

 definite line, drew attention to the fact that, in many of the lead- 

 veins of North- West Yorkshire, the sides of the veins were often 

 fluted and ' slickened ' in a horizontal rather than in a vertical direc- 

 tion. The frequent occurrence of earthquakes within or on the 

 margin of areas of volcanic activity seemed to the speaker to suggest 

 some connexion of these tremors with igneous intrusions. 



Dr. Strahan said that he joined in the discussion on this difficult 

 subject with diffidence, but that he had been interested in hearing 

 a paper in which shocks were not assumed to have originated 

 necessarily in fault-movements. He considered that that theory 

 had been pressed too hard ; certainly in some cases the wrong 

 fault had been credited with the effect, while in other cases faults 

 which had no existence had been invented for the occasion. More- 

 over, no account had been taken of the fact that faults belonged to 

 two wholly different classes. Normal slip-faults, which frequently 

 occurred in pairs as trough-faults, resulted in an expansion of 

 the tract that they traversed, each fault representing a horizontal 

 gain proportionate to its hade and throw. Upwards of a mile had 

 been gained in certain regions, where complete knowledge of the 

 character of the faults had rendered measurement possible. But 

 in other regions overthrust faults prevailed, and had resulted in 

 horizontal contraction. No distinction had been made between 

 these different classes, nor had any reason been assigned why one 

 only, out of a series of closely related faults, should suddenly display 

 activity. It had become customary to call in any fracture which, 

 happened to be conveniently situated, whatever its character and 

 whatever its age, to do duty as the ' originating fault.' Moreover, 

 the tracing of the isoseismic lines was founded on evidence which 

 was not wholly convincing. He welcomed, therefore, the alternative 

 explanation of the displacements observable after shocks, which 

 had been so lucidly advanced by the Author. 



Prof. Watts quoted the coincidence of the Leicester Fault with 

 the anticlinal fault of Charnwood Forest. Dr. Davison inferred that 

 the hade of this fault must have shifted from one side to the other 

 along the course of the fault. Of the Charnwood-Forest faults, the 

 only one of which the hade would be likely to shift along its course 



Q. J. G. S. No. 257. c 



