Tol. 55.] THE TOESIOX-STRUCTUEE OF THE DOLOMITES. 569 



Mendola Dolomite. Structurally, it is a ' crush-breccia ' (Bonney) 

 •dragged along \^dth the intrusive dyke-material of the fault, and 

 practically defining the outcrop of the fault-zone. 



Brightly-banded and black, splintery, shaly masses are present 

 on the south side of the fault-dyke, passing into greenish and 

 whitish, crumbling ' paper-shales ' in actual contact with the intru- 

 sive rock. This soft rock-material is a baked and altered repre- 

 sentative of a thinly-bedded series of bituminous limestones, in 

 which, at no very great distance, remains of the local Wengen flora 

 and fauna are to be found. 



A strip of characteristic light-grey limestone with siliceous 

 •nodules ^ intervenes at another part of the fault between the dyke 

 and the wdde outcrop of crumpled strata on the southern slope of the 

 hill. The latter are the lower or ' Wen gen ' group of the Wengen- 

 Cassian Series, and they contain abundant plant-remains, together 

 with Halohia Lommeli, Posidonomya ivengensis, and some badly- 

 preserved Arnmonites. The fossils show^ various degrees of com- 

 pression and distortion. Tufaceous grits, black earthy tuff's, and 

 thin lavas are interbedded with the fossiliferous strata. 



Considerable patches of Wengen rocks occur caught in with the 

 dyke-rock and lying in its midst, within the fault-zone. The 

 results of contact-alteration and fault-shear are evidenced in the 

 finest detail in those carried wedges. One fairly large wedge is 

 represented on the map, situated where the Pitzculatsch fault-dyke 

 widens out on the middle levels between Plon and the Pass height. 



The fault-zone may be said to fork in two directions here, 

 one branch twisting sharply north-north-eastward, in the direction 

 of Spitz Kofi, the other branch continuing the main longitudinal 

 direction east-north-eastward. The twisted branch is marked by the 

 presence of sheared and brecciated rock chiefly composed of rock- 

 masses and fragments belonging to the higher Muschelkalk horizon. 

 This ' fault-rock ' is brought against the various horizons of the 

 Werfen and Lower Muschelkalk strata exposed in the Plon anti- 

 cline on the west, and against the main fault- dyke and patches of 

 Wengen strata on the east. The direct branch is marked by the 

 presence of the porphyrite-dyke, which is here brought into contact 

 on both sides with disturbed and altered Wengen strata. 



The Yallbach fault-dykes and sills. — A glance at the 

 map (PI. XL) will show that other dykes branch out from this 

 median area on the western slope of the Pass. These are dykes 

 extending in an east-south-easterly to west-north-westerly direction 

 towards Wolkenstein. Thick masses of the characteristic fault- 

 breccias and altered tuff-rocks are present between the dykes, as 

 well as unaltered but highly-tilted remnants of Lower Muschelkalk, 



^ The * siliceous limestone ' belongs to a thin group of strata which marks 

 in this district a limiting horizon between the ' Mendola Dolomite ' and the 

 Wengen Series. It represents the ' Buchenstein ' horizon proper, and has in 

 the more recent pubhcations been referred to the Upper Muschelkalk. 



