OrdovicicDi Sedimeiits. 65 



One or two round grains with very high refractive index and 

 birefringence were noted, possessing a strong honey-yellow colour,, 

 are probably monazite, although the distinction from rutile is 

 doubtful. 



Pyrrhotite, arsenopyrite, chalcopyrite, pyrite and gold were 

 detected. The first four were evidently unacted on by the acid 

 for some reason. The gold occurs in two or three irregular 

 grains, and is quite evidently not detrital. Even after the grind- 

 ing which the gold would have received during crushing, it 

 appears quite crystalline, while one grain is thin and skeleton- 

 like, as if it had occurred in a mineral which had been dissolved 

 by acid. This inclines the writer to the view that the gold was- 

 included in pyrite, and on solution of this latter, was left as the 

 minute grains noted — the largest is not greater than .2 mm., 

 diameter. It is a well-known fact that throughout Bendigo- 

 pyrite carries gold often in considerable quantities. It may be 

 here noted that this gold could not have been included during, 

 crushing, sieving or panning, as the disc crusher was first thor- 

 oughly cleaned with pure silica, the sieves and pans also thor- 

 oughly cleaned. The writer is convinced the gold was inherent 

 in the sample. 



Magnetite was detected in minute grains, evidently having, 

 escaped separation by the electro-magnet by reason of its small 

 size. 



Ragged plates of deep brown biotite, colourless muscovite, and. 

 greenish chlorite were detected, and were probably brought down, 

 by some of the heavy minerals during flotation. 



3. Structural Alterations and Metamorphism. 



The structural alterations of the Bendigo rocks are wholly- 

 dynamic — the development of cleavage in the more argillaceous- 

 sediments with the production of slates. No shales or mud- 

 stones are represented, all having been converted into slates. As 

 these become more arenaceous, however, the cleavage is less 

 developed, until in the true sandstone there is no evidence of it 

 whatever. These structural changes are certainly a result of tTie 

 same intense forces that brought into existence the peculiar 

 regular and acute folding so typical of the area. Although 

 the Ordovician is intruded by numerous monchiquite dykes (gen- 

 erally in the neighbourhood of the anticlinal axes), there has 

 been no alteration of the walls of country rock. This is prob- 



