of the Buchan District 23 



Some additional support is afforded to the third alternative 

 by observations which I have made as to the results of 

 digestion of thin slices of the Diabase porphyrite in concen- 

 trated hydrochloric, and for periods varying from twenty-four 

 hours to a week. The porphyritic felspars were only slightly 

 affected, even after the longer period, being still tolerably 

 clear and not having lost their characteristic polarisation. 

 The microscopic felspars in the ground-mass seemed to be 

 absolutely unaffected by this treatment. Although not 

 much reliance can be placed upon such experiments, in so 

 far as they may be supposed to indicate the variety of 

 felspar present, yet in this case it is evident that a difference 

 exists between the porphyritic and microscopic felspars. 

 The latter being not all affected are doubtless nearer to 

 albite than the former. 



It seems to me, therefore, on these grounds, that the third 

 alternative agrees best with the data before me, and that 

 the porphyritic felspars may be regarded as andesine, and 

 the microscopic felspars as albite, in which the potassa 

 felspar in its triclinic form is isomorphuous with the soda 

 felspar. The constitution of a nominal andesine is albite 10 

 to anorthite 1 "0 ; but here the optical measurements suggest 

 an andesine, say, of albite 10 to anorthite 1/2 or more. In 

 this discrepancy I incline, considering the difficulties in the 

 way of obtaining satisfactory optical measurements, to give 

 most weight to considerations drawn from the chemical 

 analysis. 



Supported by the foregoing arguments the following may 

 be taken as the composition of the Diabase porphyrite of 

 the Snowy River. The secondary ores of iron are calculated 

 as magnetite, and the bastite as enstatite. 



Per cent. Molecules. 



Felspars ... 60-95 ... 180-40 



Augite 13-50 ) 75 . 00 



6-80/ '" /0 U 



10-62 ... 18-32 



•37 ... -99 



or felspar, pyroxene, magnetite, in the proportion of nearly 

 10.-4.-1. This shows the great preponderance of the 

 felspars over pyroxene, which is one of the characteristics of 

 Diabase porphyrite. 



In order to compare this rock with others of the Diabase 

 group, I have calculated twenty-eight analyses, as given in 



D 



Enstatite 

 Magnetite 

 Apatite 



