32 Notes on the Diabase Rocks 



The ratios between the minerals, as disclosed by this 

 calculation of the analysis, are as follow : — 





Per cent. 



Molecules. 



Eelspar 

 Augite 

 Magnetite ... 



58-82 

 21-05 

 1304 



173-52 



80-24 

 19-64 



or felspars, augite, magnetite, as nearly 9 - 4* 1*. 



This ratio indicates a Diabase rock in which the felspars 

 are not so preponderating as in the Diabase porphyrite of 

 the Snowy River. The ratios between the felspars are the 

 following : — 



Albite, orthoclase, anorthite, as 3* 1* 2", nearly. 



As there were no reliable measurements from which to 

 form an opinion as to the probable character of the small 

 porphyritic felspars, I can only follow the considerations 

 which influenced me in the former case. If the larger 

 felspars are regarded as normal andesine of the com- 

 position albite 1" to anorthite 1", then there remain 

 over potassa felspar and soda felspar in equal proportions, 

 which may be regarded as a potassa-bearing albite, exist- 

 ing as the microscopic triclinic felspars of the ground- 

 mass. 



There can be, it seems to me, no reasonable doubt that 

 this rock, which at Murendel occurs as intrusive masses in 

 thefelsitic beds (Lower Buchan beds), is the same, under some- 

 what different structural conditions, as is found conformably 

 underlying the Buchan limestones (Upper Buchan beds) at 

 the Snowy River. 



On the western side of the Murendel River, close adjoin- 

 ing the termination of the section which I have given, are 

 the olivine-bearing igneous rocks, in which the adit of the 

 Murendel-south Mine has been driven. I have not been 

 able to find any natural sections in which the actual relations 

 between these olivine-bearing rocks and the overlying 

 marine limestones could be traced. It seemed to me useless 

 to carry out a quantitative analysis of the former. They 

 have undergone so much alteration that scarcely any 

 points of comparison with those already analysed would 

 remain. Their microscopic features I have described in 

 the papers already quoted. For the present, it must 

 remain undecided whether the olivine-bearing rocks form 

 part of the great group of Diabase rocks of the neigh- 

 bourhood, or are much altered basalts of tertiary ages, 



