Chap. Ill,] trichinopoly district — crystalline rocks. 33 



occasionally, however, it preponderates, and at Poruttagoody a band of 

 pure black mica in small scales occurs, which appears to have crystallized 

 out at the side of a vein of binary granite.* 



The granite is much intermingled with gneiss and hornblendic-schist, 

 Mixed with schistose ^^'^^i^h, in fact, preponderate even where the granite 

 ^°^ ' veins are largest and most numerous, as in the 



rock at Samiaveram. There ' is no massive intrusion of the granite, 

 but the whole band may be considered rather as a mass of veins 

 running generally in the planes of foliation of a shattered band of 

 highly foliated hornblendic gneiss, which is frequently twisted and 

 contorted in every direction and intimately interfoliated with the 

 granite.-}- 



At Samiaveram the granitic band is about 5 miles in breadth. 

 Beyond this, in a Northerly direction, the veins are small and few in 

 number, but they may be seen occasionally for about 4 miles further, 

 after which no trace of granite is found. 



North of the granitic band the country falls away slightly, and its 



^, . , .. f surface to within a short distance of the Southern 



Plains to north or 



granitic ridge. ^^^^ ^f q^q j^^i^g^ which, for about 30 miles Westward 



of the Madras road, bound the valley of the Cauvery, is covered with ' regur' 



of variable thickness. These hills, as is usually 



The hills. . , .„ r- ^ • . r- t t 



the case m hill groups of this part of India, nse 



abruptly from the plain, and when viewed from Trichinopoly or any other 



* I have seen similar instances of bands of pure naica forming walls to the veins 

 of binary granite in Cuttack : during the consoHdation of the rock, (or crystallization 

 from aqueous solution?) the mica containing the magnesia and almost the whole of 

 the uron in the original magma appears to have separated before the sihca with the 

 greater part of the potash, alumma, and lune began to crystaUize out, as quartz and 



felspar, 



+ This is the usual character of the more granitic tracts of the country which I have 

 visited. A granitic band of similar character extends for some distance along the north bank 

 of the Puniar, in South Arcot, and I have subsequently met with rocks of the same character 

 in the Sontal pergunnah of Kuadit Kurayeh, in Bengal. 



£ 



