Vol. 55-] TOESIOK-STETJCTTJRE OF THE DOLOMITES. 619 



Cassian strata are present round the base of the mountain above 

 the level of these ridges, and a good exposure is met with farther 

 north above a steep crag of Dachstein Limestone in its true position 

 with Eaibl strata at its base. This exposure proves the peripheral 

 overthrust at the west side ; it occurs at about the 2250-metre 

 contour-line, about midway between the two western summits 

 marked 2334 and 2434 metres in the map. Subordinate shears 

 also pass through this part of the peripheral overthrust, and may- 

 be traced to the divergent series of ridges at the angle of curvature 

 or south-western corner of the mountain. 



The presence of Cassian strata above successively 

 higher horizons of Upper Trias from south to north 

 proves how strong the shearing has been in the underlay of the 

 peripheral folds. The eifect is one which can be explained only on 

 the basis of differential twists in the overlay and underlay of a fold 

 — the overlay having been twisted south-westward while it was 

 thrust outward, and the underlay having been tilted steeply with 

 easterly dip and simultaneously twisted north-eastward. Thus 

 continuously higher horizons of the underlay would be brought at 

 the shear-plane into contact with continuously lower horizons of the 

 overlay. 



Although the rocks succeeding the Cassian strata above the 

 shear-plane are less disturbed than those below, they show strong 

 compression and the thinning-out of the various horizons along the 

 shear-plane. Schlern Dolomite represents a small wedge between 

 Raibl strata above and below. The Eaibl strata below occur in 

 bedded fragments, of various horizons, much as in the case of Pian 

 de Sass (p. 597). The strata comprise (a) sandstones with Myophoria 

 Kefersteinii- (h) red and violet marls ; (c)pale dolomitic sandstones 

 and breccias ; (cl) red, violet, and greenish marls and dolomitic 

 flagstones. The usual opinion about the western face of Sett Sass 

 is that the rocks are mostly rock-slips and screes, with the ex- 

 ception of the succession of Eaibl strata just quoted and the 

 Dachstein Limestone above it. 



I have devoted several days of toilsome work to the ' screes ' 

 and ' slips,' and have seen in many places clear evidence that the 

 strata had been primarily sheared and twisted into their present 

 position. Small surface- slips have taken place along original lines 

 of fault-shear, but this is a feature characteristic of the whole 

 district, and merely demonstrates adjustment along the lines of 

 weakness. 



Pralongia and Castello thrust-plane. — The exposure of 

 Cassian strata on the Montagna della Corte Pass, between the two 

 summits of Eichthofen Eiff and Monte Sief, shows another repetition 

 of the fossiliferous marly horizons and succeeding Cidaris-lime- 

 stonea. This subordinate thrust-plane may be traced eastward 

 towards Castello along a ridge of block-scree which is chiefly com- 

 posed of shear-fragments of Cassian strata and Schlern Dolomite 

 weathered in situ. The fault- plane passes east-north-eastward 

 through the meadows, as far as the transverse fault of the Yal- 

 parola stream. The Valparola Fault is a westerly one limiting 



