On the Torsion- Structure of the Dolomites. 419 



2. ' The Torsion-Structure of the Dolomites.' By Maria M. 

 Ogilvie, D.Sc. [Mrs. Gordon]. 



The paper opens with a geueral account of the work of Eichthofen, 

 Mojsisovics, Bothpletz, Salomon, Brogger, the author, and others 

 on the Dolomitic area of Southern Tyrol. It then gives the results of a 

 detailed survey recently made by the author of the complicated strati- 

 graphy of the rocks of the Groden Pass, the Buchenstein Valley, and 

 the massives of Sella and Sett Sass ; together with the author's inter- 

 pretation of these results, and her application of that interpretation 

 to the explanation of the Dolomite region in general. The author 

 concludes that overthrusts and faults of all types are far more common 

 in the Dolomites than has hitherto been supposed. The arrangement 

 of these faults is typically a torsion-phenomenon, the result of the 

 superposition of a later upon an earlier strike. This later crust- 

 movement was of Middle Tertiary age, and one with the movement 

 which gave origin to the well-known Judicarian-Asta phenomena. 

 The youngest dykes (and also the granite-masses) are of Middle 

 Tertiary age, while the geographical position of both is the natural 

 effect of the crust-torsion itself. This crust-torsion also fully 

 explains the peculiar stratigraphical phenomena in the Dolomite 

 region, such as the present isolation of the mountain-massives of 

 dolomitic rock. 



The Groden Pass area, first selected for description by the author, 

 is a distorted anticlinal form running approximately JNT.N.E. and 

 S.S.W., and including all the formations ranging from th.e Bellerophon- 

 Limestone, through the Alpine Muschelkalk and Buchenstein Beds, 

 to the top of the Wengen Series. When studied in section, the strata 

 of the Pass are found to be arranged in a complex fold form, showing 

 a central anticlinal with lateral wings, limited on opposite sides by 

 faults and flexures. Strongly marked overthrusting to S.S.E. in 

 the northern wing is responded to by return overthrusts to N.N.W. 

 in the southern wing. The strata in the middle limb of the anti- 

 clinal wings bend steeply downwards into knee-bend flexures. 

 Through these run series of normal and reversed faults, into which 

 has been injected a network of igneous rocks, giving rise to ' shear- 

 and-contact ' breccias, which have previously been grouped as 

 Buchenstein tuff and agglomerates, and referred to the Triassic 

 period. 



The area of movement of the Groden Pass system is an ellipsoid in 

 form. Two foci occur within it, where the effects of shear and strain 

 have culminated. The forces of compression acted not in parallel 

 lines, but round the area, thus causing torsion of the earth-crust. 

 Two main faults occur (with a general east-and-west trend) whose 

 actual lines of direction intersect at a point about midway between 

 the foci of the torsion-ellipsoid. These are the chief strike torsion- 

 faults ; many minor ones pass out easterly and westerly from the foci, 

 forming longitudinal or strike torsion-bundles. The strike 

 system of faults is cut by a series of diagonal or transverse 

 curved branching faults, with a more or less north-easterly or north- 

 westerly direction. These diagonal faults may cut each other, or 



