620 MISS M. M. OGILVIE [mKS. GORDON] ON THE [Allg. 1 899, 



another diagonal arch, that of Lagazuoi and Sasso di Stria, parallel 

 with the Sett-Sass and Priilongia diagonal arches. 



Westward from Montagna della Corte, the lower thrust-plane 

 may be traced through the southern slopes of Pralongia. A wedge 

 of dolomitic limestone is again present at about the 1950- and 

 2000-metre contour -lines immediately south of the Pralongia 

 summit, and is here associated with an intrusive sill of porphyrite. 

 The fault-plane curves sharply northward through the Kuones 

 meadows, still accompanied by the intrusive sill, and gradually 

 assumes a higher angle of inclination (70° to 80°). Its further 

 continuation is round a north-western arc between Laugs-da-Piir 

 and Siadu, with downthrow to Siadu. The intrusive sill round 

 Pralongia is therefore a thread, from the Pescosta and Langs-da-Pur 

 iutrusive mass. 



The Priilongia-Castello thrust-plane describes a wider torsion-arc 

 round the south-west than the higher thrust-plane above the 

 Richthofeo Reef, but it is characterized by the same A & B spiral 

 movements which are found to answer generally for the southern 

 torsion-arcs in this neighbourhood. The Castello portion represents 

 the B-spiral, where the underlay has been tilted with a steep inward 

 dip and twisted northward and north-westward, while the overlay 

 has been twisted almost horizontally outward to south and south- 

 east. The Pralongia portion represents the A-spiral, where the 

 underlay has been steeply tilted with a north-easterly dip and twisted 

 in that direction, while the overlay has been twisted south-westward. 

 The overlay of the Pralongia overthrust comprises the strata of the 

 Pralongia ridge, which on the other side of the ridge form the 

 underlay of the peripheral overthrust of Sett Sass. Therefore the 

 strata of the Pralongia thrust-whorl have endured twining strains 

 of the same nature as those described on a preceding page for the 

 main whorl of the Sella Massive. 



The normal fault of Monte Sief. — The flexure of the 

 Monte-Sief strata towards the north is very marked. The Wengen 

 rocks strike north 40° east, and dip 35° north-westward. The flexure 

 is cut by a highly-inclined normal fault, with downthrow towards 

 Sett Sass. As the angle of inclination is much greater than the 

 inclination of the reversed fault-planes of Montagna della Corte and 

 the Pichthofen Reef, the three fault-planes would meet if continued 

 upward. Thus the tendency of compression was to bring the over- 

 thrust mass of Sett Sass always closer to the Middle Triassic 

 anticline of Monte Sief and Col di Lana. 



Once more it is the softer Wengen-Cassian Series which has been 

 chiefly crumpled, sliced, and torn, together with small shear-frag- 

 ments of the overlying calcareous rock, between the advancing 

 overthrust mass of Upper Trias and the upwardly-bulged mass of 

 Middle Triassic limestone (see fig. 10, p. 586). 



The normal fault of Sett Sass. — The Schlern-Dolomite 

 rock of Sett Sass, which has a scarcely perceptible dip (about 8° to 

 10°) above the Richthofen Reef, makes a steep flexure on the north 

 side of the mountain, cut by a slight flexure-fault. The Raibl and 



