Lr. M. If. Ogilvie Gordon — Monzoni and Upper Fassa. 311 



' Asta ' fault-sill — the western or Predazzo portion having been 

 subsequently displaced southward, and downthrown relatively to 

 the Monzoni portion. Again, the ' eruptive fissure ' has been 

 described as "probably " continuing through aporphyrite fault-dyke 

 east of Lago di Selle and passing E.N.E. through the Costabella 

 range. This is, however, not the case. The east-west fault holds 

 its own direction, passing due east into the Carapagnazza. I regard 

 it as one of the leading Alpine faults in this district, comparable with 

 the Groden and Bucheustein faults which I have described in my 

 former papers. Its age may be far greater than the injection of the 

 monzonite, seeing that the injection was synchronous with the 

 superposed ' Asta ' movement. Moreover, the term ' eruptive ' was 

 applied under the conception that Monzoni was a volcanic centre 

 in Triassic time, the monzonite being the deep-seated facies of 

 Triassic surface flows of porphyritic lava and tuff. This I hold to be 

 a mistaken conception, as I have carefully examined all the supposed, 

 surface flows of porphyrite in the vicinity of Monzoni and in Upper 

 Fassa, and have found them to be intrusive sills and dykes, injected 

 subsequently to the main intrusion of monzonite, at that epoch in 

 fact when the monzonite fault-sill was cross-sliced, and its western 

 portion laterally displaced as far as Predazzo. According to my 

 mapping of this district there are no igneous contemporaneous rocks 

 later than the quartz porphyry except the tuffs in the Wengen- 

 Cassian beds. 



Having now pointed out how some of the older misconceptions 

 regarding Monzoni may have arisen, I shall proceed to indicate 

 briefly a few of the leading observations I have made throughout 

 the district. 



The Miocene Age of Porphykite Sills. 



Fig. 1 is a generalized transverse section through the Campagnazza 

 Meadowland and the Costabella range. It shows how the two 

 crust-slices, the Campagnazza and the Costabella slices, are them- 

 selves cut by subordinate fractures. These are * Asta ' fractures, 

 striking either E.N.E. or W.N.W. ; the inclination of all shear- 

 planes, is northward, but the angle of inclination varies very con- 

 siderably. The chief tectonic complications occur in the immediate 

 proximity of the older east- west fault. In the eastern or Fuchiade 

 part of the Campagnazza, a small segment of older strata is at this 

 fracture overthrust above Upper Werfen strata. In the middle part 

 of the Campagnazza, several porphyrite fault-sills ascend at this 

 shear-zone. The loestern or AUochet portion near Monzoni, like the 

 middle area, shows several shear-planes and porphyrite fault-sills, 

 but the strata dip more steeply. The porphyrite sills run con- 

 tinuously from the ' Asta ' faults into virgating cross-faults directed 

 N.N.W.-S.S.E. and N.N.E.-S.S.W. Two virgating groups of faults 

 and. fault-dykes divide the Campagnazza into the three segments 

 which I have indicated — the Allochet segment on the west, the 

 middle or main cross-segment, and the Fuchiade segment on the 

 east. The middle segment is an upthrow relatively to the segments 



