Vol. 55.] TOESION-STRUCTUEE OF THE DOLOMITES. 611 



towards the height of the Groden Pass, the other converging west- 

 ward round the Champinoi slopes below Lang Kofl.. (c) Intrusive 

 sills penetrate the fault-planes in the opposite bundles, and are 

 continuous with the cross-dyke. The sill-intrusions are pre- 

 dominant in the inclined planes hading north in the Vallbach area 

 of downthrow towards the Spitz Kojl group north of the Groden Pass, 

 and in the inclined planes hading south in the Champinoi area of 

 downthrow towards the Lang Kofl group on the west side of the 

 cross-arch, (d) The cross-axes of the buckle are directed east-north 

 east and west-south-west (from the Groden-Pass apex of virgating 

 faults to the Lang-Kofl apex), and north and south. Another 

 example of a torsion cross-buckle occurs at Pescosta. 



Owing to the inelastic nature of the crust, deformation of the 

 east-and-west folds was trho essential condition upon which new 

 crust-forms could be carried out. The virgating fault-curves which 

 turn away from the cross-buckles on all sides show that these 

 areas were individual centres of evolute movements in 

 deep layers of the crust. The term 'evolute' has special 

 reference to a compensating ' involute ' movement (p. 608). The 

 intrusion of igneous rock into the cross-buckle shows that the 

 molten layers below the crust had involute movement towards 

 such centres. The greater the compression at any given area, the 

 wider would the buckle become and the greater would be the mass 

 of intruded rock. 



The northern synclinal curves of Sella have reference to these two 

 cross-buckles on the Groden-Pass Anticline ; while the southern 

 synclinal curves of Sella have reference to two other cross-buckles 

 formed at the areas where the same diagonal anticlines (Sella 

 Pass and Campolungo Pass) have been superposed on the Belvedere- 

 Buchenstein Anticline. 



A general law may be thus stated : — The chief torsion-buckles 

 in a system of torsion-folds are formed where the cross-anticlines 

 are superposed upon main anticlines of the pre-existing series of 

 crust-folds; such areas of superposition are of necessity major 

 centres of crust-weakness, peculiarly liable to invasions of molten 

 rock during the active period of crust-torsion. 



Torsion as affected by, and affecting, petrographical 

 conditions. — All round Sella the Wengen-Cassian Series has been 

 let down by faults from the anticlinal cross-buckles, and has been at 

 the same time twisted downward into the imderlay of the peripheral 

 overthrust of the massive. The series has therefore sunk relatively 

 to both the great masses of calcareous rock — Middle Triassic lime- 

 stones below and Upper Triassic limestones above. The intermediate 

 Wengen-Cassian Series would inevitably have been buried at some 

 parts had the twisting and compression been greater (see fig. 20, 

 p, 609). 



The same danger threatened the mixed series of Jurassic strata, 

 comprising marls, breccias, and thin nodular limestones. They 

 were bent inward and twisted downward beneath portions of 

 Dachstein Limestone moving outward and upward. Curious 



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