580 MISS M. M. OGILVIE [mRS. GOEDOJST] ON THE [Aug. 1 899, 



4. Partial intersection of the opposite fold-arcs. — An area of 

 curve-intersection of the opposite wings is present, which fairly corresponds with 

 the central area of the Pass, and the upper part of the eastern slopes. The 

 southern wing cuts off the exposure of part of the southern arc 

 of the northern wing. Presented graphically, the fault-figure of inter- 

 section is like the figure 8 laid lengthways across the central area of the 

 Pass, while the virgating faults branch outward from the two ends of the figure. 

 As a matter of fact, the area of fold-intersection is marked on the ground by 

 strongly-contorted rocks, by block-structure, by convergence of the ramifying 

 threads of intrusive rock, and by frequently twisted cleavage — a combination of 

 features sufficiently indicative of ' interference' crust-strains. 



5, Cross-folding in its relation to torsion-forces. — Regarded 

 from this central intersecting area, the strike of the radiating fore-folds and 

 back-folds is directed to four points of the compass : — west-north-west, west- 

 south-west, east-north-east, and east-south-east ; and as the strike of a fold is 

 rectangular to the forces producing it, we have to distinguish four directions 

 from which forces were acting : — south-south-west, north-north-east, north- 

 north-west, and south-south-east. They may be tabulated as follows : — 



West side (northern wing), strike Forces from north-north-east 



west-north-west and east-south-east. and south-south-west. 



East side (southern wing), strike Forces from north-north-east 



west-north-west and east-south-east. and south-south-west. 



East side (northern wing), strike Forces from north-north-west 



west-south-west and east-north-east. and south-south-east. 



West side (southern wing), strike Forces from north-north-west 



west-south-west and east-north-east. and south-south-east. 



As the axis of the Groden Pass Anticline itself lies slightly west-south-west 

 and east-north-east, the forces from north-north-west and south-south-east 

 have been stronger here than those from north-north-east and south-south-west. 

 The phenomena of the converse curves of the opposite folds, the convergence 

 of the anticlinal fault-blocks towards the median area of the Pass, and the 

 divergence of the fault-blocks outward into more and more oblique positions, 

 indicate that the forces did not act in straight lines across the 

 area, but in some form of curves round the area, after the 

 manner of force-couples. Clearly, also, therelati-^e action of the forces 

 must have been inward in the median area where the maximum compression 

 is presented, and outward on the lower slopes where virgation is presented. 



The following problem may be constructed in order to elucidate the probable 

 action of the forces as indicated by the author's obseryations. 



Imagine an almost straight line to be drawn across the Pass nearly east and 

 west from Pescosta to Plon. Bisect this line by another at right angles. These 

 divide the region roughly into four parts or quarters, which may be represented 

 by A, B, C, & D in the accompanying diagram (fig. 8, p. 581). Let C & D be 

 pushed northward, A & B be pushed southward, so as to form an anticline in the 

 meridional direction. At the same time, or subsequently, imagine a motion of 

 the nature of a twist to be given to each of these quartei-s : A & C to be moved 

 counter-clockwise, B & D clockwise. The effect will be, therefore, as if every 

 part within each quarter had received a combination of a forward motion and 

 a twist; the resultant motion will be of the nature of a spiral for each quarter 

 (epitrochoid). 



Additional complications are indicated, such as a tilting of the southern 

 quarters D & C, I) rising and C being depressed, the raised portion D being 

 pushed so far above A in the central area. The chief fact, however, is that 

 the phenomena of the Groden Pass indicate the combined action of a 

 push and a twist — crust-compression and crust-torsion — resulting in con- 

 tending spiral whirls. 



We have, as it were, two dissimilar sets of force-couples acting on the Pass 

 anticline, the more powerful set being north-north-west and south-south-east, and 

 causing counter-clockwise torsion, the subsidiary set being north-north-east and 

 suoth-south-west, and causing clockwise torsion. Consequently, there are two 



