602 MISS M. M. OGILVIE [mRS. GOEDON] OIST THE [Aug. 1899, 



of north-north-east and south-south-west faults, with westerly down- 

 throw, which mark the east side of Sella. 



The higher reversed fault-branch, above Cra di Mont, may be 

 traced as a low overthrust-plane round the curvature of the moun- 

 tain as far as Piz Kofl. It represents the return-fold from Sella in 

 the south-eastern quarter of the Groderi-Pass torsion-system. The 

 thrust-plane separates the outward-dipping Schlern Dolomite of the 

 Piz terrace from numerous small twisted segments of the Schlern 

 Dolomite and Cassian strata belonging to the underlay of the peri- 

 pheral fold. The dip in these segments is always inward, and they 

 correspond to the similar fragments on the Campolungo Pass and 

 Corvara Col. It will be objected by some that these segments are 

 all landslips and do not permit of a stratigraphical reading. To 

 such objections I can only reply that minute investigations on the 

 spot will convince an unbiased mind that the ' slips ' are sheared 

 slices through the middle limb and underlay of a twisted fold.^ 



The lower reversed fault, above Crap de Sella, has a high incli- 

 nation and crosses the Groden Pass almost vertically in a north-west 

 to south-east direction. The summit of Crap de Sella is composed 

 of the highest horizons of Cassian strata, — thick-bedded, yellowish 

 limestones full of Cidaris-sj)ines and encrinite-stems. A few block- 

 remnants of dolomite weathered in situ show that the Cassian Series 

 was followed by the conformable succession of Schlern Dolomite. 

 Thus the Crap de Sella fault has effected a repetition of Cassian 

 strata and Schlern Dolomite at a level of about 250 feet below the 

 outcrop of the same horizons of rock on the Piz slopes, and is the 

 lowest of the reversed faults on this side of Sella. 



The overlapping of shear-planes in different horizons of the 

 crust in the Bova-Alp region, as contrasted with the divergence 

 ^ of shear-planes in the Piz region, is a typical torsion-phenomenon, 

 and points to relatively stronger oblique compression in the Bova- 

 Alp region. It may be compared with the case in the northern 

 fold-arc of the Buchenstein Valley, where the shear-planes overlap 

 in the west and open out in the east of the centre of torsion. 



The steep slopes below the ' Crap ' present a complex network 

 of faults in the Cassian and Wengen strata. The general tendency 

 is to buckle up the strata along diverging torsion-curves parallel with 

 the curve of the Piz terrace above. The faults are chiefly north- 

 and-south, and they meet the main north-north-east and south-south- 

 west diagonal fault of the Campolungo Pass. The curve described 

 by the course of the Pudort stream precisely corresponds with 

 the general direction of these torsion-curves. 



The northern and southern fold-arcs in the peri- 

 pheral over thrust. — A continuous series of peripheral over- 

 thrusts has now been traced completely round the Sella Massive. 



^ The complete form of a torsion-curve is composed of a number of unit- 

 shears ; a shear in one direction dies out, as a shear in a new direction starts at 

 a small angle with the other. Thus a curve of torsion-shears is formed on the 

 principle of furcation previously enunciated (p. 581), and the precise angle of 

 inclination differs in the several shear-planes. 



