421 



GEOLOGICAL NOTES. 



By T. W. Edgeworth David, B.A., F.G.S. 



(1) Note on the Laccolites of the Junction Mine, neak 



Mandurama. 



The Junction Mine, near Mandurama, has long been famous 

 for the remarkable character of its auriferous deposits. These 

 have been described by previous writers as ore beds, on account 

 of their comparatively low angle of dip and general conforma- 

 bility with the bedding planes of the sedimentary strata, with 

 which at first sight they present the appearance of being inter- 

 stratified. A recent examination, however, of these deposits by 

 the author in company with Mr. G. A. Stonier, geological sur- 

 veyor, has convinced him that these deposits are not true ore 

 beds, but laccolites. 



Omitting the basalts and their underlying gravels, the author 

 thinks that three distinct varieties of rock are developed at this 

 mine, each probably belonging to a different geological date. 



(1) First and oldest are claystones showing remarkable even- 

 ness and regularity of bedding, felspathic layers alternating with 

 more siliceous, while on certain horizons are intercalated thin 

 beds of limestone. The author is indebted to Professor W. J. 

 Stephens, F.G.S. , for the information that Pentavwrus occurs in 

 these limestone beds on Mr. Rothery's run, not far from the 

 Junction Mine, and also for the suggestion that continuations of 

 these beds have been replaced by the laccolitic eruptive rocks at 

 the Junction Mine, a conclusion which the author had pre- 

 conceived before communicating with Professor Stephens, as a 

 result of observing the abundance of calcite in the undecomposed 

 portions of the laccolites, and the occurrence of a small bed of 

 partly silicitied limestone at the Belubula Mine, just below the 

 Junction Mine. 



