^®-^' 55'] TOESION-STRIJCTTJEE CF THE DOLOMITES. 603 



[n all cases overthrust has been outward from the heart of the 

 mountain, and has served to neutralize steep inward and downward 

 flexures of the strata from the twisted anticlinal buckles which 

 surround the synclinal area of the mountain. Hence, no matter 

 where a section is drawn through Sella, the same general lines will be 

 obtained in the peripheral fold-form (figs. 15 & 17, pp. 592 & 594). 

 The fold-form at once recalls the ' fan-structure ' of the central 

 massives of the Alps. 



Cassian strata have been frequently carried along the overth rust- 

 plane above Schlern Dolomite, while the Schlern Dolomite below 

 the overthrust has been variously cut, as a continuous slice of irre- 

 gular thickness in the north-west of the mountain, and as sheared 

 wedge-shaped fragments at other parts of the periphery. 



The peripheral overthrust-fold subdivides itself naturally into two 

 fold-arcs, a northern and a southern, separated by a well-marked 

 depression in the massive, from Pian de Sass westward across the 

 central area. The northern arc is the wider and larger ; it com- 

 prises the Meisules torsion-curve round the north-west of Sella, and 

 the Piz-terrace torsion-curve round the north-east, overthrust being 

 to the north-west and north-east respectively. The strata of the 

 underlay thin out southward, in wbich direction therefore con- 

 tinuously lower horizons are sheared against the thrust-plane. The 

 Meisules and Piz-terrace curves correspond to the D & C force- 

 spirals in the Groden-Pass scheme of torsion. 



The southern fold-arc comprises the Pordoi torsion-curve round 

 the south-south-west and the Bova-Alp curve round the south- 

 south-east. This curve represents the sunken northern fold-arc 

 corresponding to the A & B force-spirals in the Buchenstein-Yalley 

 torsion-system. Overthrust has taken place to the south-south- 

 west and south-south-east. The underfolded strata have been 

 dragged and sheared into a series of slices thinning out northward, 

 and therefore continuously lower horizons occur next the thrust- 

 plane as it twists towards Yal la Stries and Bova Alp. 



As these peripheral overthrusts have taken place in oblique 

 compass-directions, the axis of maximum tension in the rocks com- 

 posing the massive is a line dividing the D & A force-spirals on 

 the west from the C & B force-spirals on the east. Such an axis 

 would run, generally speaking, north and south from the Pissadoi 

 Ravine to Monte Porca, immediately east of the Pordoi summits. 

 The resultant forces of compression must have acted at right angles 

 to this axis (see fig. 19, p. 607). 



Considered from the aspect of the torsion-movements, the Sella 

 Massive is a buckle occupying a torsion-basin between the cross- 

 anticlines of the Sella Pass and the Campolungo Pass, and is itself 

 directed north-north-east and south-south-west. But it occupies a 

 synclinal area between the east-and-west anticlines of the Groden 

 Pass and of the Belvedere ridge. P'rom this it would seem that 

 there has been oblique folding in the Sella Massive, north-north-east 

 and south-south-west, as well as meridional folding. These two 

 fold-axes are denoted by the direction of two chief ravines in the 



