596 MISS M. M. OGILVIE [mKS. GOKDOn] ON THE [Aug. 1 899, 



thickness of about 1100 feet. The overthrust-plane may be said to 

 disappear from view when it enters the Cassian strata of the Sella 

 Pass. Considerable tectonic disturbances are, however, evinced in 

 the Wengen-Cassian group wherever exposures are found on the 

 southern slopes of the Sella Pass towards the Salei stream, and on 

 the steep descent of Yal la Stries towards E,oja. The soft strata 

 are twisted in all directions, and riddled with small faults. This 

 is the explanation of the wide surface- outcrop attained by Wengen 

 and Cassian strata, more especially the former, on the south-west 

 side of the Sella Massive. 



The contorted Wengen-Cassian strata and the sheared fragments 

 of Schlern Dolomite below the thrust-plane form part of the anti- 

 clinal group of strata belonging to the south-western quarter of the 

 Groden-Pass torsion-scheme, while the strata above the thrust-plane 

 represent a back-fold from the correlated part of the Sella syncline. 

 Differential movements of torsion have taken place here in super- 

 posed horizons of the earth's crust, the upper horizons having been 

 twisted northward and north-westward, while the lower horizons 

 were twisted southward and south-eastward. The shear-plane has 

 passed spirally through the Wengen-Cassian Series of softer strata 

 and also the more resisting Schlern Dolomite. 



The torsion of the thrust-fold into an arc-shape is cognate with 

 the torsion of the strike of the rocks. Thus the Meisules fold-arc 

 forms a southern curve in the virgating group of torsion-curves 

 demonstrated on the west of the Groden Pass. The heaping-up of 

 the calcareo-dolomitic strata towards the north and north-north-west 

 is correlated with a general attenuation of these strata in the internal 

 portion of the massive, and a consequent sinking of Jurassic strata 

 along a curve parallel with the curve of the Meisules fold-arc. 



The Pordoi Pass overthrust. — The Pordoi Pass forms 

 what is usually regarded as the southern limit of the Sella Mountain. 

 It extends east and west between two wide outcrops of the Wengen- 

 Cassian Series, here, as at the Groden Pass and the Sella Pass, 

 strongly contorted and often faulted. 



The Pordoi Spitze are southern summits of the Sella Massive 

 which stand north of the Pass and exhibit an undisturbed succession 

 of Schlern Dolomite, Paibl, and Dachstein-Limestone strata dipping 

 slightly outward. On the south side of the Pordoi Pass the summit 

 of Sasso Pitschi (Sasso Beccie) rises from the midst of the Wengen- 

 Cassian Series. It is composed of Schlern -Dolomite rocks, not only 

 dipping steeply north-westward, but also showing actual twists in 

 the strata similar in direction to twist-curves which can be seen in 

 the Dachstein Limestone that forms the highest summit (Boe) in the 

 massive. The disturbed stratigraphy of Sasso Pitschi offers a 

 marked contrast to the gentle outward dip of the strata on the 

 opposite Pordoi summits. 



The differential dips are separated by the Pordoi Pass overthrust- 

 plane, which hades northward : I have described the details on 

 a previous occasion.^ Sasso Pitschi presents a lower slice of 



1 Geol. Ma?. 1894, pp. 53-54 & fig. 



