14 W. S. DUN. 



which dips very gradually under the plain. The mines are 

 contained within a sedimentary and igneous complex which 

 shows the effect of variable regional metamorphism. 



The trend of the main Broken Hill Lode sympathises in 

 the main with that of the associated country rocks, and 

 along this portion of the area the greatest rock alterations 

 appear to have taken place. Thus the sediments appear 

 to have been sands and clays with a high proportion of 

 felspar, which in the neigbourhood of Broken Hill, the 

 Pinnacles and Round Hill lodes, have been altered, in many 

 places to sillimanite schist, whereas the sediments appear 

 as sandstones, mica schists, and allied types well away 

 from these lodes. 



Gneisses of variable types, but closely related to each 

 other, occupy much of the area near the main lodes, but 

 they are conspicuously absent at a distance from the 

 central area. Amphibolites and pegmatites are extremely 

 common in the Broken Hill district, the types of the central 

 area being different, in some measure, from the types lying 

 to the east and west. All these intrusive rocks are in the 

 form of sills or laccolites. A subordinate amount of basic 

 igneous material occurs in the form of cross dykes of later 

 age than the amphibolites. The amphibolite sills them- 

 selves are later than the gneissic rocks. The Broken Hill 

 lodes were formed within the core of an old mountain range 

 long since denuded to a plain. 



Near the main lodes well marked zones of crushing occur. 

 Cross-zones of crushing occur also as well as the main 

 zones following the strike of the country rocks. Both 

 types of crush zones contain pegmatite and quartz intrus- 

 ions. The main crush zones appear to lie along the trend 

 of strike faults, but these have not been determined satis- 

 factorily at present. One of the large cross faults, how- 

 ever, has been worked out by Mr. W. R. Browne of the 



