[Proc. Koy. Soc. Victoria, 34 (N.S.), Pt. I., 1921.] 



Akv. VIII. — The Petrology of the Ordovician Sediments 

 of the Bendigo District. 



By J. A. DUNN, 13.Sc. 

 (Howitt Natural History Research Scholar, 1920). 



[Read 14th July, 1921.] 



1. Introduction. 



The Ordovician sediments form practically the only rocks 

 actually represented in Bendigo, and outcrop over almost the 

 whole area except where occasionally covered by shallow allu- 

 vium. The structure of the series has been so thoroughly 

 described by numerous geological workers in the past, particu- 

 larly E. J. Dunni and F. L. Stillwell,^ that no description is 

 here needed. One or two points may however be noted. 



An exhaustive examination of the graptolites obtained from 

 different parts of the field has shown that the Lancefield, Bendigo 

 and Castlemaine zones of the Lower Ordovician are represented 

 here, but there is, however, no lithological difference in the re- 

 presentatives of these three zones. There is every gradation 

 between a typical sandstone and a typical slate, and these are 

 the only representatives of the original sediments. The fresh 

 slate has a dark to light bluish-grey colour, the sand- 

 stone a dark to light grey shade. On weathering this 

 is altered to a buff colour in both cases, the slates being 

 generally darker than the sandstone, except where the latter 

 have been almost entirely replaced by limonite. The limonite 

 staining is derived from the decomposition by meteoric waters 

 of the pyrite contained in the fresh rock, and replaces the clayey 

 material, and constitutes the more important cement of the 

 weathered rock. Where, however, the importance of the limonite 

 as a cementing medium is small, the sandstone becomes a soft, 

 porous, crumbly sandstone, and the slate a fine greasy fissile 

 material. 



2. " Report on the Bendigo GoWfield," Nos 1 and 2. E. J. Dunn, Geo). Surv. Vict. 18%. 

 2. " Gold Deposition in the Betidiyo Goldfield," Parts I., II., and III. F. L. Stillwell. Bull 4.8 

 and IC. Adv. Council Sc. and Industry. 



