2S Notes on the Diabase Rocks 



places at least by masses or wide bands of a dark-coloured 

 basic rock, more finely crystalline than those I have just 

 described. The section terminates at a high hill of massive 

 felsite on the western side of the Murendel River. This hill 

 appears to have been an original protuberance round which 

 the Middle Devonian marine limestones have been laid 

 down, and which still enclose it on three sides. Imme- 

 diately to the south of this hill Buchan limestones have 

 either been originally laid down upon, or have been broken 

 through by the olivine bearing rocks, in which the adit 

 of the Murendel-south Lead Mine was driven. I have 

 referred to these rocks in a former paper* I was then 

 inclined to regard these olivine bearing rocks and the 

 intrusive dyke-like masses beyond the Murendel mine as being, 

 in fact, the same. I now propose to add some further par- 

 ticulars resulting from more recent investigations. My 

 remarks refer now to the dj^ke-like masses near the Murendel 

 mine, and along the track, as shown in the section accompany- 

 ing this paper. The extreme freshness of appearance of this 

 fine-grained rock, and its occurrence as an intrusive dyke of 

 considerable size in the Lower Buchan beds might lead to the 

 suspicion that it is comparatively recent in geological age 

 as compared with the Diabase porphyrite of the Snowy 

 Biver. It even bears some resemblance, when examined in 

 thin slices, to some of the miocene tertiary volcanic rocks of 

 the Dargo High Blains. I hoped that by examining it 

 carefully both by microscopic and chemical analysis, I might 

 be able to form an opinion not only as to its position in the 

 petrographical system, but even to hazard a conjecture as 

 to its geological age. These data might then also bear upon 

 the question whether the olivine-bearing rocks of Murendel 

 south are intrusive or contemporaneous. I selected a 

 sample for examination from the dyke-like mass crossed by 

 the track immediately before reaching the Murendel mine. 



The examination of several thin slices showed me that 

 the rock is composed of a ground-mass consisting of : — 



(a) A little pale, brownish yellow to almost colourless 

 basis. In some slices this basis is almost absent. 



(b) Minute felspar prisms in great numbers. These appear 

 to be all triclinic, and in some slices show flow structure 

 very perfectly. Where most numerous, they are also often 

 arranged radially, or are clustered round augite grains. 



(e) Grains and ciystals of magnetite. 



* Report of Progress Geological Survey of Victoria, Part V., p. 117. 



