316 Dr. M. M. Ogilvie Gordon — Monzoni and Upper Fassa. 



they have locally broken through the thrust-masses as fault-sills and 

 dykes, brecciating and altering the strata, the igneous and breceiose 

 material thus replacing the strata along the strike of the thrust- 

 masses. Accordingly I dissent from the opinion advanced by 

 Mojsisovics and repeated in slightly different form by Salomon, 

 that the igneous rocks of Bufaure, Belvedere, etc., constituted 

 a volcanic Triassic facies of the Marmolata limestone. The massive 

 sheets of porphyrite at Bufaure, etc., are wholly intrusive and run 

 into radial and strike fractures associated with the Asta-Judicarian 

 movements. The stratigraphy of the district will be fully set forth 

 in the geological map and sections that are to accompany my 

 complete paper. 



Block-Poephyrite. 



Some of the most puzzling problems concerning the mineralogy 

 of the district are associated with the block-porphyrite (' Block- 

 Porphyr ' of Mojsisovics), or, as it has been frequently termed, the 

 conglomerate or ' agglomerate ' structure of the igneous rocks. The 

 explanations offered by all who have surveyed in this district except 

 myself have been based upon the assumption that the Bufaure and 

 neighbouring sheets of porphyrite were contemporaneous Triassic 

 lavas. During my own field-work I have had to make a close 

 investigation of various sills of block-porphyrite, and find that the 

 conglomeratic appearance is due to various causes. While in many 

 cases the block-structure is simply a result of original segregation 

 in concretionary lumps or lenticular patches and bands, there are 

 others where the structure is superinduced. I found it necessary to 

 determine the joint-cleavage systems in the porphyrite very care- 

 fully, as the later, more basic injections have frequently been injected 

 along the joint-planes, and so encircle isolated masses of an earlier 

 sill, just as the sill-magma itself enwrapt or infiltrated particular 

 masses of strata when it was injected. I classify the different types 

 of block-structure as follows : — 



(a) Original agglomeratic structure dependent upon various forms 

 of segregation products ; 



(b) Joint-hlock structure due to decomposition and weathering of 

 the porphyrite along intersecting curved joints ; 



(c) Superinduced breceiose structure where a sill has after con- 

 solidation been subjected to differential shearing ; 



(d) Combination- sill structure, where intermittent injections have 

 passed into the same fault-zone, and the later injections have broken 

 up the older. 



The Sella Pass. 



The north wing of the Buchenstein anticline occupies a crust- 

 zone in which old eruptive fissures of Triassic age occurred. I have 

 previously determined the Buchenstein-Mahlknecht fault-zone ex- 

 tending east and west through this area, and have pointed out that 

 the tufaceous as contrasted with the calcareous facies developed in 

 the proximity of, and north of, this Triassic fracture-zone during the 



