IN THE MYSORE STATE. 3 



II.-PETROLOGICAL CHARACTERS. 



It has already been stated that the magnesite is usually dis- 

 tributed as white veins through the altered peridotite. But in the 

 present instance, the ferro-magnesian carbonate, which proves to be 

 breunnerite, occurs as dark-grey, well crystallized, porphyritic 

 crystals, measuring from half an inch to an inch across, lying in a 

 dark-greenish-grey, fine-grained matrix of talc and picrolite. The 

 rock has a most deceptive appearance in the field and in hand- 

 specimens, and those who have handled the specimens have 

 invariably been surprised when told the true nature of the porphyritic 

 crystals and of the matrix. The iron-staining on weathered surfaces 

 give the porphyritic crystals the appearance of pyroxenic or 

 hornblendic crystals, and the greenish-grey matrix, being a good 

 imitation of the diabase type of rock, completes the deception. 



Although each phenocryst is shown in section and by cleavage 

 to be a single large crystal, its outline is seen in microscopic section 

 to be ragged and irregular where it abuts against the matrix. Magne- 

 tite, in the form of minute granules, or as dust gathered into patches 

 and short bands, occurs in the breunnerite phenocrysts as well as 

 in the matrix. Occasional shreds of talc and picrolite occur also as 

 inclusions in the breunnerite. 



The matrix consists of interlaced colourless talc and pale-green 

 picrolite, together with numerous granules of rhombohedral carbonate, 

 the magnetite-dust referred to above (with an occasional lump 

 showing crystal outlines), and a few granules of pyrite. Analysis 

 shows the presence of chromium, but I have not been able to detect 

 with certainty the mineral chromite in any of the thin sections. 



The specific gravity of a large specimen of the rock was found to 

 be 2*95, whilst that of the matrix was 2-845, an d OI the breunnerite 

 phenocrysts, 3168. The matrix and breunnerite are thus in the pro- 

 portion of— 



Breunnerite , . . .35 per cent. 

 Ma tnx 65 „ ~,, 



( 3 ) 



