294 



GEOLOGICAL SURVEY 



south-east of Karur, on the banks of the Aiyar, and south of 

 Namakal. These will some day furnish large supplies of very 

 handsome marbles of various colors. 



Specially interesting to the mineralogist are the great 

 magnesite deposits to the north of Salem and at foot of the 

 Shevaroy Hills with their associated serpentine, chrysotile, 

 chalcedony, and chromic iron. The magnesite occurs in veins 

 traversing talcose, chloritic and hornblendic rocks, which have 

 been greatly altered and decomposed. 



Former observers, including Captain Newbold, had describ- 

 ed these very singular deposits, but their origination by the 

 action of thermal waters was first made out by Mr. King 

 and the author. Other similar hotspring deposits were noted 

 and described by them as occurring at various places in Salem 

 and Trichinopoly Districts. The chromic iron occurring at 

 the Chalk hills appears to occur in strings in the decom- 

 posing serpentinous rocks. It was formerly mined by the 

 Porto Novo Iron Company. 



The corundum-yielding rocks in the south-western part of 

 Salem District have not yet been examined by the Geological 

 Survey. They lie beyond the tract surveyed. 



In 1861 Messrs. C. 2E. Oldham and King took up the 

 examination of the " Diamond Sandstones " of Malcolmson, a 

 great series of sub-metamorphic or transition rocks occupying 

 the greater part of Kadapa and Karnul Districts, and ex- 

 tending into North Arcot, Bellary, Nellore, and Kistna Districts 

 and the Nizam's territory. These rocks, the area of which is 

 extremely well defined in most places, its boundaries coinciding 

 with the great orological features of the country, had attracted 

 much attention before, and various speculations had been 

 hazarded as to their position in the geological scale. It had 

 long been known to contain the demantiferous beds, but the 

 stratigraphy of the great series had not been worked out. 



