NILGHIRI HILLS. 229 



a ternary compound of quartz, felspar, and mica, all largely crystallized, 



and the rock is consequently very easily decom- 



Accidental minerals. , j i i rpu i it, • i 



posed and broken up. ine only other mmerals 



hitherto observed in it are Magnetic Iron, Garnets, and Beryls, both 



. . , IT „ of the opaque and aqua-marine varieties. These 



Aquamarines at Jian- r i i 



sy*™' latter occur at Kangyam in the Coimbatoor 



district, and an excavation was formerly opened by Mr. Heath for the 

 purpose of obtaining them. The works have been now abandoned, 

 and the excavations are filled with water. 



With regard to the age of this Granite little more can be said than of 

 Age of Granite un- that of the Trap. To judge from the appearance 

 °°^^°' of the veins at Sunkerry Droog more especially, 



the gneiss must have been rigid at the period of the intrjasion 

 of the granite, and it would seem, moreover, that the country must 

 have undergone some dislocation, since, although tolerably numerous 

 in the neighbourhood of Matepolliam, no trace of them is to be found 

 on the hills, the gneiss of which must, therefore, have been elevated above 

 the sphere of their penetration. It is therefore desirable to defer the 

 further consideration of this subject, until the low country of Coimbatoor 

 and the plateau of Mysore have been carefully investigated. 



Dislocations and Mineral Veins. — It is necessarily somewhat diffi- 

 cult to establish satisfactorily the amount and 



Difficult to determine t .• /• ii i- i i' i • i i , i 



dislocations in Gneiss direction 01 the dislocatioQs which have taken 

 '^°^° ^^' place in a country consisting solely of foliated 



rocks, inasmuch as the only evidences even of the existence of 

 such dislocations are to be found in the effects produced by them on 

 the physical features of the surface, and, in comparatively rare instances, 

 in the presence of mineral veins. \ Evidences of both kinds are fortu- 

 nately to be found in the Neelgherry country, sufficient to enable us at 

 all events to ascertain the direction of those great lines of faulting 

 which have determined the form of the principal hill masses under 

 consideration. 



