Mineralogy of Southern India. 207 



vided into prisms by cracks. In the hill of Palicondah, near 

 Vellore, it is very common ; it is never associated with quartz 

 rock, but generally occurs where euritic granite forms hills 

 of any size. 



Schorl quartz Porphyry. 

 This is a rare and very beautiful rock in Southern India : 

 it is composed of crystals of black schorl, embedded in a 

 paste of homogeneous and translucent quartz. As I believe 

 it is not known in Europe, at least it has not been defined 

 by any author I have met with, it is therefore one of 

 those rocks for which I have been obliged to invent names. 

 To avoid loading the science with mere unmeaning words, 

 I have proposed to take Dr. Macculloch's definition of " Por- 

 phyry," in a generic sense, as a rock composed of crystals of 

 one mineral, embedded in a continuous non-crystalline paste 

 of another. Thus the definition of this rock becomes " Black 

 schorl in quartz porphyry ;" and by dropping the preposition, 

 and as I know of no schorl in India which is not black, we 

 have " schorl quartz porphyry," which is surely a practical de- 

 fining term, sufficiently convenient for use, without requiring 

 any great jaw-deflecting exertion. It is easily distinguished 

 from hornblende by the appearance of the crystals, which 

 are of a jet black colour; neither lined on the surface, nor 

 imperfectly tabular. The fracture of the crystals, irregular 

 and conchoidal, and resplendent and shining. They are very 

 brittle, and yield easily to the knife : melt very easily before 

 the blowpipe, and intumesce in a peculiar manner, by which 

 they can at once be recognised, even when very minute 

 indeed. This rock is very different from what Boase calls 

 " schorl rock," which he distinctly defines to be an aggre- 

 gation with " crystals of quartz" is found South of Paul- 

 wole, about twenty-five miles South of Ryacottah, and in the 

 " Trap Dyke formation of Mallapanbetta" (which vide here- 

 after,) in which it forms beautiful rocks with crystals of schorl, 

 an inch nearly in thickness. It seems also to have been 



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