Vol. S5-] 



TOESION-STRUCTrEE OF THE DOLOMITES. 



609 



the summit-buckle, but similarly elongated in shape with reference 

 to a north-north-easterlj and south-south-westerly axis. 



The mountain-massive of 8ella may be described as an elliptical 

 basin or sag. 



Eock-twining and dolomitization. — The main body of 

 Dachstein-Raibl-Schlern-Dolomite rocks is disposed as an elliptical 

 'tier' of rock which may be termed a rock-whorl. In each 

 part of the whorl corresponding to an individual spiral movement 

 the upper horizons of the whorl have been twisted and bent in 

 opposite directions from the lower horizons. The mass has 

 therefore undergone twining-strains to a very great 

 degree. The lines dividing parts of the rock- whorl which move 

 in opposite directions are ' nodal lines ' in respect of the crust- 

 movements. Dynamo-metamorphic changes undoubtedly have taken 

 place in rocks undergoing such powerful strains, and the processes 

 which brought about the dolomitization of the 'Dachstein' and 

 ' Schlern ' calcareous masses may have been directed or greatly 

 aided by these torsion-strains. 



Fig. 20. — The Sella folds, before and after torsion. 

 N. ■ ' S. 



Character of the two movements. — The effect of the earlier 

 movement has been described above as a simple wrinkling of the 

 crust into a series of folds with east-and-west strike (p. 605). 



I have distinguished five folds in the region examined : — 



1 {a) Groden-Pass arch ; (6) Grroden-Pass trough, now involved in the 



thrust-plane of the northern fold-arc of the Sella Massive. 



2 {a) Arch of Meisules-Pissadoi-Zehner summits and the Lago di Boe 



escarpment ; {b) corresponding trough through the heart of the 

 mountain, including the Jurassic infold. 



3 {a) Boe summit ; {h) trough between Boe and Pordoi. 



4 (a) Arch of Sella-Pordoi-Ool di Stein summits ; {b) southern trough, 



including Val la Stries, Monte Forca, Bova Alp. 



5 (a) Cima-di-Rossi portion of the Belvedere-Buchenstein arch. 



The later movement was locally one of compression from west- 

 north-westerly and east -south-easterly directions. Had there been 

 no previous folds the result would presumably have been the for- 

 mation of crust-folds directed north-north-east and south-south-west. 



Q.J. G. S. No. 219. 2e 



