3^ CRETACEOUS ROCKS OF S, INDIA. [PaRT II. § 1. 



outlier to the South-east of the hill country of Northern Trichinopoly, 



lie the oldest rocks of the sedimentary series, the 

 Terany Hill ; plant-beds. 



fossils of which (with some doubtful exceptions, 



plant remains,) afford somewhat ambiguous evidence of their age. 



Some miles further to the North another small detached hill or ridge, 

 Gneiss of NortliemTri- that of Nedduvassel, rises almost at the border of 

 cliinopoly and s. Arcot. ^-^^ Cretaceous rocks ; and beyond this point the 

 narrow gneiss plain, 4 or 5 miles across, which intervenes between the hill 

 country and the former rocks, widens out rapidly, and the hills trend 

 away to the North-westward, and the Cretaceous rocks to the North-east, 

 until, at Sadras, the whole extent of the Payen Ghat, where not covered 

 by recent alluvia, consists of the old metamorphic rocks. Over a great part 



of this area the gneiss is very uniform in character. 

 Its mineral character. 



It is a hard, tough, compact rock, in which 



quartz and felspar predominate, but containing a, variable proportion of 



hornblende, and frequently much garnet. It also contains, perhaps 



universally, a small amount of magnetic iron, although this can scarcely 



be detected in a hand specimen. This mineral is 

 Magneticlron in Gneiss. i i • 



found as sand, accumulated m black streaks on 



the bed of every nullah in the countiy that drains Crystalline. rocks, and 



not unfrequently in those from the sedimentary rocks, especially the 



coarse, half consolidated, sands of some portion of the Cretaceous seiies. 



In the Salem district, but especially in the Ahtoor gap, this mineral h^ 



been long known to occur in broad strings of the massive ore, and has 



been worked by the natives from time immemorial, for the manufacture 



of some of the best iron and steel produced in India or elsewhere. But 



this region is far distant from any of the now existing sedimentary rocks, 



and we must attribute the ferruginous element of certain beds of these 



latter to the local segregation of iron from some less concentrated source, 



such as the ordinary gneiss of the country would supply in its abraded 



material. 



