DESCRIPTION OF THE PRINCIPAL EXPOSURES. 183 



special section and further details of the geology of the immediate 

 neighbourhood of Salem will be published in a separate memoir. 



The charnockite series are represented in other hill masses in the 

 Salem District, for instance in the Javadis and in the Dharmapur 1 

 Hills. In the Dharmapuri taluk corundum occurs, with sillimanite, 

 rutile, biotite and cryptoperthite, as constituents of large 

 ellipsoidal inclusions near the junction of the charnockite series 

 with a biotite granite which occurs in large quantities in this 

 taluk and that of Hosur. As the Salem and Coimbatore Districts 

 are now being carefully surveyed, it is advisable to postpone further 

 discussion of the local geological relations of the charnockite series 

 until they have been worked out in greater detail. 



Coimbatore District. 



The charnockite series exposed in various parts of the Coimbatore 



District will be described by Mr. Middlemiss, who has been surveying 



this area for some four years. But one instance illustrating 



the amphibolization as well as one of the 



"ne^rTirrupur" 15 wa y s in which banding is produced, should 



be referred to at once. Near Tirrupur railway 



station there are considerable exposures of hornblendic gneisses along 



the banks of the Noyal river, which, on microscopic examination, are 



found to contain hypersthene, and in other ways to repeat the essential 



features of the charnockite series. As sections shew various stages 



in the amphibolization of the pyroxene with consequent production 



of green hornblende, the unusually large quantity of the latter mineral 



is satisfactorily accounted for. Garnets, more often than hornblende, 



are found amongst the products of the alteration of pyroxene in the 



South Indian charnockites, but it now seems likely that, whilst the 



unstable pyroxene may give rise to garnet at high temperatures, 



hornblende is the more usual result of change at lower temperatures. 



It is thus likely that one of these minerals may characterise one area 



or one mass which happens to be deeply buried, whilst the other may 



arise when the rock becomes deformed nearer the surface. 



F ( 65 ) 



