Tol. 55.] TOKSTON-STETJCTTJEE OF THE DOLOMITES. 597 



Schlern-Dolomite rock below the shear-plane, while the Cassian 

 strata on the Pordoi Pass are above the shear-plane. Thus the same 

 relations hold between Sasso Pitschi and the Pordoi summits as 

 between the rocks of the Griiner Pleck and the Meisules group in 

 the north-west of Sella. One important fact is that in every part 

 of the circuit so far Wengen and Cassian strata have 

 preserved precisely the same palaeontological and 

 petrographical features. 



The direction of the torsion-movements associated with the Pordoi 

 overthrust is indicated by the change of strike and dip in the over- 

 thrust group on the east and west sides of the Pordoi summits. On 

 the west side, towards Val la Stries, the Schlern Dolomite strikes 

 almost due north-and-south^ and dips from 5° to 10° inward. A small 

 transverse fault east of the Pordoi summits marks a sharp curvature 

 of the strike to west-north-west and east-south-east, and it then 

 turns gradually eastward, while the dip is as much as 15° to 20° 

 outward. The Sasso Pitschi fault-block has likewise a rapidly 

 twisted strike, from west-south-west and east- north-east opposite 

 Monte Porca to south-west and north-east a little lower down the 

 Cordevole slopes. The dolomite-rocks of Sasso Pitschi show abun- 

 dant evidence of shear-brecciation and slickensides. 



The strike-torsion therefore describes an arc from west to east 

 round the southern curve of the Sella Massive, comparable to the 

 torsion-curve round the south of Cherz Hill opposite Arabba 

 (p. 585). The shear-plane is also similarly inclined in the two 

 places. These features indicate that the Pordoi portion of Sella 

 represents a fold-arc in high horizons of the northern wing of the 

 Buchenstein torsion-system. At the same time it is a sunken block 

 occupying an area of tension between the two opposite and divergent 

 torsion-curves of the main anticline, namely, the northern or Varda- 

 Cherz curve and the southern or Belvedere-Cima di Eossi curve. 

 The fold-arc has been broken, and the overlay and underlay of the 

 fold have been twisted differently. The shear-zone occurs chiefly 

 in the Wengen-Cassian Series, while the Schlern-Dolomite rocks 

 that form Sasso Pitschi represent a torsion-wedge caught in an area 

 of cross-movement and sharp curvature. 



Eecognition of rocks on the east side. — The eastern 

 side of Sella, facing Campolungo Pass and Cherz Hill, proved an 

 arduous field to examine. It is much cut up by faults, and the 

 greatest care has to be exercised in identifying the rocks belonging 

 to each fault- segment. The chief difiiculty is in respect to the age 

 of the dolomitic rock, as the Dachstein-Limestone horizon is in great 

 measure dolomitic, and can be distinguished from Schlern Dolo- 

 mite only by a persevering search for fossils. Fortunately Megalodon 

 triqueter, the typical Dachstein fossil, occurs very numerously 

 throughout the Dachstein rock-horizons in most parts of Sella, and 

 the higher Dachstein horizons are full of smaller bivalves ,and 

 _gasteropods. 



The Raibl strata are easily recognized from their petrographical 



