Dr. M. Ogilvie Gordon — The Langkofl Thrust-mass. 489 



but the surface is vraved and bent, and the outcrop of the plane is 

 frequently interrupted by small cross-faults. The plane is essentially 

 a crush-plane, at which the older group of rocks has been thrust 

 westward. 



On the north side of the Groden Valley the thrust-plane continues 

 through the Pitsch-Berg, where the Werfen or Lower Triassic Strata 

 have been overthrust westward above the Middle Trias. Further 

 north rise the Dolomite peaks of the Geissler group, whose geological 

 relations and 'reef-like' form Mojsisovics compared closelv with those 

 of the Langkofl Massive {Bolomit-Eiffe, 1879, pp. 210-11). The 

 band of Miischelkalk Rock at the base of the Dolomite in both 

 these Massives reaches the height of 2200 met. contour, whereas 

 in the Triassic succession of the Grrtiden Valley below St. Christina 

 it is exposed at the 1400-1500 met. contours. The explanation, 

 however, is provided in this important overthrust of the rocks of the 

 Upper Groden area, and the subsequent flexures and faults which 

 the thrust-mass has undergone. 



Near St. Christina, I detected in three of the stream-cuttings on 

 the south side of Groden Valley the same thrust - plane, almost 

 horizontal, passing through the Wengen lavas and tuffs, and 

 duplicating that horizon and the Buchenstein Strata ; and I traced 

 it westward where it underlies the Seiser Alpe. Thus the Langkofl 

 or Schlern Dolomite thrust-mass simply rests upon another thrust- 

 mass on the west. At a still lower level another thrust-plane is 

 present, passing through the Buchenstein and Mendola Dolomite 

 horizons, and with overthrust again of Augite, Porphyrite, and 

 Buchenstein rocks. 



The Upper Groden thrust-mass is therefore composed of subjacent 

 thrust-slices of varying thickness and importance. The thin shear- 

 slice of portions of Werfen, Muschelkalk, Buchenstein, Porphyrite, 

 and Wengen tuffs emerging from below Langkofl on the north side 

 owes its position to an east-west dislocation within the thrust-mass. 

 The thrust-planes below and above it are apparently the same as 

 those above and below the lower segment of Schlern Dolomite and 

 Wengen - Cassian Strata in Sella Massive. This thin shear - slice 

 underlies the thick development of Schlern Dolomite in both 

 Massives, which represents the main thrust-segment. In the Sella 

 Massive it is succeeded by another thrust-slice at the Boe summit, 

 where Eaibl and Dachstein horizons have been overthrust above 

 Upper Trias and fragments of Jurassic and Cretaceous Strata. The 

 conformable succession of Schlern Dolomite upon Upper Cassian 

 horizons belonging to the thrust-mass is well preserved on the 

 southern and eastern flanks of the Langkofl Massive. 



The similarity of the geological relations in the Langkofl and Sella 

 Massives proves the essential continuity of the important thrust-plane 

 at the base of the Schlern Dolomite. The varying dip of the plane 

 to north or south or in other directions, both in Sella and Langkofl, 

 is due to superimposed plicational and dislocational effects ; while 

 the position of the overthrust Cassian Strata and Schlern Dolomite 

 relatively to different stratigraphical horizons in the underlving shear- 

 slice indicates the previous existence of folds in the rock-masses cut 



