^"ol- 55-] TOESION-STRUCTTJRE OE THE BOLOMITES. 605 



rocks of the eastern side ; they are intersected by planes at varying 

 oblique angles to north-west in the north-eastern curve, and to 

 south-west in the south-eastern curve. 



Evidence of a former east-and-west strike.- — Minor 

 complications are present on the eastern side which bear testimony 

 to a pre-existing strike. These are east-and-west flexures and 

 flexure-faults. The Lago di Boe escarpment, in the middle of the 

 eastern side, is an upthrow separated from two shelving terraces 

 north and south by east-and-west faults. The northern is the Piz 

 terrace, with flexure to the Grodeu Pass ; the southern terrace is 

 that of Pian de Sass, which is separated from another upthrow, 

 namely, that of Col di Stein, by an east-and-west fault parallel with 

 those which limit the Lago-di-Boe upthrow. A southerly flexure 

 of the strata descends from Col di Stein towards the Buchenstein 

 Valley. 



Thus the eastern side, taken from north to south, is composed of 

 alternating arches and troughs. Such a series indicates crust- 

 folding at some time from north and south. 



The same succession of east-and-west flexures is quite apparent 

 throughout the mountain, but the component arches occur at wider 

 intervals in the central downthrow of the mountain. Corresponding 

 to the Pian-di-Sass trough, a broader trough runs through Yallon 

 and the Pizza Longa to the inner basin of the west side of the 

 mountain. The arch of the Lago-di-Boe escarpment is continued 

 westward through the areas of the Pissadoi and Meisules summits. 

 The Col-di-Stein arch, south of Pian de Sass, curves southward 

 and runs through the Pordoi and Sella summits. The troughs 

 external to these arches are indicated by the outward flexures, but 

 the actual hollows of the outer troughs have been covered by the 

 peripheral overthrust. 



There is absolute stratigraphical evidence in all cases that this 

 series of east-and-west flexures in Sella Mountain has been broken 

 up by the various oblique shear-planes, the radiating faults and 

 joints, and the cross-cleavages which I group together under the 

 name of torsion-phenomena. Therefore we have to bear in 

 mind that the forces which induced these phenomena had an already 

 folded crust to work upon. 



Stratigraphy at the Boe summit. — There is still the 

 highest terrain of the Sella Massive to be discussed, which should, 

 according to Mojsisovics's description in his ' Dolomit-Riffe,' be 

 wholly occupied by ' Dachstein Limestone.' That is, however, not the 

 case, as I showed in my paper in the Geological Magazine for 1894. 



Ascending to the Boe summit from Yal la Stries, the Dachstein 

 horizon is met at a lower level than that mentioned by Mojsisovics. 

 Abundant specimens of Megalodon triqueter were found by me 

 in the rock at the 2600-metre contour-line. The strike was 

 north-west and south-east, the dip very slight, about 8° or 10° 

 towards the summit. Already from that horizon the eye was 

 attracted by the appearance of bright, brick-red, earthy strata on 

 the north side of the Boe summit. Yiewed from a distance they 



