Vol. 65.] . CALIFORNIA^ EARTHQUAKE OP 1906. 19 



It seemed probable that San Pedro corresponded to an anticlinal 

 axis in course of formation, Santa Catalina to a syncline, and 

 San Clemente to another anticlinal ridge. 



Pressures great enough to produce these foldings must act at 

 right-angles to the orientation of these islands, that was, north- 

 north-eastwards. When stresses acting in' that direction encoun- 

 tered obliquely the great fault ranging from Point Arena to the 

 Mohave Desert, they naturally must have a tendency to be relieved 

 by an horizontal displacement of the proximate lip in a direction 

 away from the acute angle of incidence formed by the direction of 

 •the stresses and the strike of the fault, in the way illustrated by 

 the Author's diagrams and experiments. 



Mr. L. H. Cooke said that Mr. Collins's suggestion, as to the 

 possibility of ' heaves ' having been formed by a relative displace- 

 ment of the two walls along their strike, deserved special attention 

 in the cases where the walls were slickensided, fluted, and striated 

 horizontally, as was most usual in the lead- and zinc-mines in the 

 Mountain Limestone of Flintshire and Derbyshire according to his 

 (the speaker's) observations. In these districts, the vein-fissures 

 had mostly small ' throws,' and in some instances might possibly 

 be merely enlarged joints in which the sides and filling had been 

 slickensided by repeated to-and-fro movements — a view which 

 seemed in consonance with what had been written by Sir Archibald 

 •Geikie. The common allegation that the flutings and striations of 

 ■slickensides indicated the direction of dislocation had not, so far 

 as he knew, been proved: it seemed a survival of the regrettable 

 practice of confidently putting forward plausible assumptions as 

 proved facts. Since, however, the precise trigonometrical survey 

 in California had demonstrated movement or dislocation along the 

 strike of the fissure, it was very desirable to scrutinize these 

 Flintshire and Derbyshire veins more closely in this respect. 



Attention had been previously called to apparent strike-dislo- 

 cations in highly disturbed rocks (as, for example, in the Lewisian 

 Gneiss), but in such districts some alternative explanation was 

 nearly always possible, and geologists would doubtless be grateful 

 to the trigonometrical surveyors of California for presenting them 

 with incontrovertible proof of such movements. 



The century-old J. C. L. Schmidt rule, which postulated a descent 

 • of the hanging wall, applied, so far as he had been able to test it, 

 on the lines first indicated by Christian Zimmermaun, to the great 

 bulk of heaves in other districts with which he was acquainted. 



The President remarked that the Author fully admitted that 

 local earthquakes might arise from tectonic dislocations, and his 

 conclusions only applied to great world-shaking seisms. But, even 

 if it were found impossible to trace the San Francisco earthquake 

 to a particular line of fracture, it by no means followed that the 

 cause was not to be found in a sudden release from a state of strain. 

 The system of forces applied to the material used in the mould 

 would give rise to a fracture inclined at 45° to the direction of the 

 slit which represented the San Francisco fracture. The subject 



c2 



