Notes on Movchiqviic Dyhes. 13 



above or below. Circulation of solutions may operate throufrhout the 

 whole field, but for enrichment it is necessary that : — 



(1) The solutions should traverse the dyke channel. 



(2) The solutions should be gold-beax'ing. 



(3 The solutions should meet material which will precipitate the 

 gold. 



Only where these three conditions meet will enrichment result. En- 

 richment will thus not necessarily result when a decomposed dyke 

 meets a quartz reef or spur. If the enriching solutions are travelling 

 downwards, enrichment is only likely in the shallower levels of any 

 mine. 



The fourth possible method is suggested by the example recorded 

 by i\rr. Dunn (2) in the Hercules and Energetic Mine, where a quartz 

 spur cut through the dyke, and is rich in the dyke and poor on each 

 side. No evidence has been produced to show how the dyke could 

 possibly so operate. 



In concluding this paper I should like tn acknowledge the invaluable 

 assistance of Mr. T. W. Itoss, B.M.E., Assistant Inspector of Mines, in 

 gaining for me access to the mines, and of Mr. H. S. Whitelaw and the 

 members of his staff, and by no means least the unfailing courtesy of 

 the mining managers. In the laboratory I have been much indebted 

 to Professor Skeats and Mr. Summers, M.Sc. To Mr. H. S. Grayson I am 

 especially indebted fnr the preparation and staining of rock slides. 



Summapy. 



The dykes of the Bendigo goldfield are intruded into folded ordo- 

 vician rocks. They form a parallel system with a parallelism coinci- 

 dent with the strike of the rocks. They occur along the course of each 

 anticline, and in the mines are always approximately in the centre 

 country. At the surface they are always decomposed and soft. 



Specimens of fresh lava were obtained from several of the mines. 

 These were examined, and one was analysed. The rock was found to 

 be highly basic, and a member of the monchiquite group. The mon- 

 chiquite was found to pass into camptonitic varieties at the Forbes 

 Carshalton Mine, and at the Goldfields No. 1 shaft. A kersantite is 

 recorded from the One Tree Hill Mine to the south-west of Bendigo. 



The age of the rocks can only be determined by petrological analogy, 

 and is considered to be probably mid-kainozoic 



The relation of the lavas to the distribution of the gold is discussed. 

 Evidence has been obtained in the Ironbark Mine to suggest the 

 secondary deposition of the gold. Gold is found more abundantly 

 and irregidarly in thoroughly brecciated material than in the adjoining 



