Mineralogy of Southern India. 201 



kind ; and as this rock is decidedly compound, I have pre- 

 ferred considering it as a compound of Diallage ; because 

 Macculloch (" Rocks," page 648) says, that Diallage occurs 

 in Shetland of a black colour, and forms a compound rock 

 with felspar, of a brown colour. As Macculloch most likely 

 never saw the Diallage rock of Shetland polished, it is very 

 probable that there is little difference in the colour of the 

 fracture, which he calls dark grey and brown, and I have called 

 bluish, lead colour black, so as to maintain some ideal resem- 

 blance to the colour of the fracture of black granite. To 

 prove the composition of a compound rock is a difficult 

 matter, unless it can be freely examined in sites : this I had 

 no opportunity of doing ; but Capt. Newbold visited Tauvi- 

 cairy, where the stone is said to have been quarried. I have 

 not his remarks at hand to refer to ; but from some spe- 

 cimens sent me by Dr. Cole of Madras, I may be allowed to 

 doubt, if he saw the real rock ; as those specimens, though 

 very small, seemed to be a variety more like Serpentine, and 

 certainly much more Magnesian than the black Diallage. 

 Chemical analysis also of a compound rock is not a certain 

 guide; but as it affords considerable assistance I publish the 

 following quantitative analysis, though with some diffidence, 

 as the analysis of a Magnesian rock is not an easy job, while 

 our means in India are very imperfect, and I have not yet 

 had leisure to make a good one. The rock, powdered, was 

 a " grey white" colour. The analysis, heated red-hot, lost 3-40 

 per cent, of weight ; but ignited for fifteen minutes more, lost 

 4-73 per cent. It had been found that caustic potash did 

 not decompose the analysis readily, therefore mixed with 

 carbonate of soda, and ignited ; dissolved in muriatic acid, 

 evaporated to dryness, &c, acidulated, and water added, and 

 filtered to separate the silica. The filtered solution was 

 precipitated by excess of ammonia, and allowed to stand 

 a night to separate manganese : it was then filtered, and 

 evaporated to near dryness, diluted again with water and 



