612 MISS M. M. OGILVIE [MRS. GORDOlJf] OX THE [Allg. 1899, 



Tariations in thickness were elBfectecl, both in the case of the AVengen- 

 Cassian Series and the Jurassic rocks. Such appearances have been 

 attributed to natural causes — ground-inequalities during deposition, 

 coral-growth, etc. ; but any striking cases observed at Sella are 

 results of the complex shearing movements during crust-torsion and 

 compression. 



While the crust-strains can be alwa3's demonstrated in the 

 fragmental rocks hj means of the contorted and fractured strata, 

 less stratigraphical disturbance is visible in the calcareous horizons. 

 Sufficient proof, however, of crust-strains through the calcareous 

 rocks is presented by the significant fact of their dolomitization. It 

 may bo added that the earthy Eaibl strata which occur beween the 

 two calcareous horizons of Upper Trias very frequently show 

 contortions and fractures when the calcareous horizons seem com- 

 paratively little disturbed. 



Strike -torsion. — The fault-blocks composed of the lower 

 geological horizons and extending between the old anticlines and 

 the later diagonal anticlines have a small torsion-angle relative to 

 the original strike. For example, taking the D-quarter of the 

 Groden Pass as a type (PI, XL), we find that the Muschelkalk block 

 of Pitzculatsch Hill has a torsion-angle of strike of about 20°, while 

 the Upper Triassic rocks in the massive have a greater torsion-angle, 

 about 35° to 40°. It is only in the highest horizons that the 

 strike-torsion has reached an angle of 50° to 70°. 



The Groden-Pass scheme will show how two cross-directions of 

 strike are always co-ordinated as a necessary result of the action of 

 the torsion-forces. A west-south-west and east-north-east strike at 

 one part of a torsion-curve is responded to by a west-north-west and 

 east-south-east strike in the other part of the curve ; similarly, a 

 south-west and north-east strike is responded toby a north-west and 

 south-east strike in the other portion, and a south-south-west and 

 north-north-east strike is co-ordinated with a north-north-west and 

 south-south-east strike. 



Summary. — Briefly summarizing, we note that 



(1) The synclinal basin of Sella is included within a polygon of faults whose 

 planes incline inward. 



(2) Steep downthrow- faults or flexures from the surrounding anticlines are 

 neutralized by peripheral overthrvists from the synclinal basin. The Wengen- 

 Cassian Series is sunk, strongly contorted and mutilated between these 

 corresponding faults round the massive. 



(3) Torsion has taken place in different directions in the overlay and underlay 

 of the peripheral overtbrust. 



(4) The same horizons of Dachstein strata that bend outward in the peri- 

 pheral overtbrust bend inward to form the underlay of a summit-overthrust, 

 but tbe underlay has been twisted in opposite directions from the twisting in 

 the overlay. The overlay of the summit-overthrust repeats the same directions 

 of twist as the overlay of the peripheral overthrusts. 



(5) Thus the Upper Trias of the massive is arranged in two Avhorls, as a 

 result of combined movements of torsion and compression. 



(6) The Lower and Middle Triassic strata are heaped up in torsion-buckles 

 situated at the four oblique corners of the massive : Plon, Pescosta, Arabba, 

 and Eodella. These have been local centres of eruption during torsion- 

 movements. 



