592 WILLIAM J. MILLER 



this exceptional occurrence than that generally adopted for those of the first 

 named category; a shding of soft sea bottom deposits on a sloping surface under 

 gravity, helped forward perhaps, if on a large scale, by earthquake shock or some 



other jolt-like impulse It follows from the conception of these structures 



that the deformation was contemporaneous, and preceded the deposition of the 

 overlying beds.' 



For the reasons below listed the writer is strongly inclined to the 

 conclusion that the corrugated zone at Gaspe developed as a result 

 of differential movement within a great block of strata while it was 

 being thrust faulted, (i) If we accept the hypothesis of subaqueous 

 sliding, it is necessary to attribute the nearly straight upper surface 

 truncating the contorted zone to erosion, but the very character 

 and uniformity of both the contorted and inclosing strata seem to 

 render this extremely improbable; (2) the worn character of both 

 the upper and lower surfaces of the contorted zone, with certain 

 beds locally missing as noted by Logan, are best explained on the 

 basis of differential movement within the mass of limestone; (3) that 

 the conditions were very favorable for differential movement is 

 borne out by the fact that the contorted zone lies not far from the 

 bottom of a large block of Devonian strata which has been exten- 

 sively thrust faulted over Cambro-Ordovician strata; (4) the strike 

 of the contorted zone is approximately parallel to the strike of the 

 thrust fault; (5) the corrugations conform to the direction of dip of 

 the inclosing beds; (6) according to Logan's section, the corruga- 

 tions developed just where the conditions were most favorable for 

 differential movement, that is, in the weakest (most shaly) part 

 of a mass of the strata 200 feet thick with appreciably more arena- 

 ceous and resistant strata above and below this mass; and (7) the 

 rather puzzling occurrence of the two nearly straight, thin beds 

 within the contorted zone (see Fig. 3) may be more reasonably 

 explained on the basis of differential movement than on the basis of 

 subaqueous gliding because, as revealed by careful examination 

 of the figure, these layers, as well as the layers capping the con- 

 torted zone, show clear evidence of having been deformed and even 

 cut across locally by the corrugations, thus indicating that the con- 

 tortions took place after deposition of those layers. 



^ Personal communication from Dr. Clarke. 



