Br. M. M. Ogilvie Gordon — Monzoni and Upper Fassa. 309 



VII. — Monzoni and Upper Fassa, 



By M. M. Ogilvie Gordon, D.Sc. 



rPHE following pages present some results obtained by me from 

 X the geological survey of the Fassa district in the Dolomites 

 comprising the Bafaure and Monzoni mountain groups, where the 

 porphyritic and monzonitic rocks are widely exposed. In selecting 

 this district I had two objects in view — (1) to study in detail the 

 tectonic relations between the igneous and stratified rocks ; (2) to 

 test the results which I had previously obtained from my survey 

 of Sella Massive, etc. 



The Tertiaky Age of Monzoni. 



The Campagnazza Meadowland stretches east of Monzoni as 

 undulating slopes descending southward from the Costabella range 

 of limestone to the Pellegrino Valley. The meadowland is com- 

 posed of Permian and Werfen strata, and I discovered in it a number 

 of intrusive sills of porphyrite associated with two faults. The 

 more northerly of the faults runs east-west at the southern base 

 of the Costabella range, and it continues westward as the northern 

 fault-limit of the Allochet ridge and Monzoni. This fault is 

 steeply inclined to the north, and the differential movements have 

 effected the relative downthrow of the Costabella fault-block on 

 the north. The other fault follows a curved strike, its course being 

 E.S.E.-W.N.W. in the Campagnazza, but curving to E.N.E.-W.S.W., 

 where it is continued as the southern fault-limit of the Allochet 

 ridge and the monzonite rocks of Monzoni. The curvature is 

 convex towards the north, and the fault-plane is inclined northward. 

 In the Campagnazza there is north of the fault a crumpled sheared 

 slice of Werfen strata ; but this block is represented at Monzoni 

 by the intruded igneous rocks. The tectonic relation of the 

 monzonite intrusion is therefore that of a fault-sill, representing 

 in the strike the Campagnazza crust-slice between the two faults, 

 and injected at an angle of strike-curvature. 



The fault-block south of the curved fault may be conveniently 

 termed the Pellegrino fault-block. It comprises quartz porphyry, 

 Permian strata, and fault-fi-agments of Werfen strata, and has been 

 driven southward, representing one of a series of overthrust slices 

 which occur south of the Pellegrino Valley (Fig. 1).^ Still farther 

 south the Cima d'Asta overthrusts are present, and as there are 

 occurrences of Jurassic and Cretaceous strata in the shear-zones of 

 the overthrust faults, it is clear that the slicing took place in con- 

 nection with Tertiary movements in the Alps. The Campagnazza 

 fault-curves were certainly not earlier than the overthrusts ; thus, 

 my geological mapping, which proves the replacement of the 

 Campagnazza sheared slice by the monzonite intrusion, proves at 

 the same time that the intrusion was of Tertiary age. I find 

 further that I can offer an interpretation of the sequence of injections 



^ I have made no personal examination of the district south of the Pellegrino 

 VaUey. 



