GEOLOGICAL FEATURES. 1$ 



and the advanced state of alteration of the constituent minerals is 

 only such as might be expected on the borders of the great intrusive 

 mass of the Nilgiri charnockite. 



At some period, probably anterior to the intrusion of this latter 

 rock, earth-movements of great intensity must have taken place 

 in south-east Waindd : these resulted in the formation of a series of 

 fissures and of innumerable faults, of varying throw, in the gneiss ; 

 the fault planes are still indicated by bands of hard, black mylonitic 

 material, resembling the bands so common in the so-called irap- 

 shotten gneiss found in so many parts of the Madras Presidency 

 and in Bengal, the true explanation of the origin of which, — vis., 

 rise of temperature and consequent fritting of the material along 

 the fault planes — has recently been given by Mr. Holland. 1 



(2) Charnockite, 



The chief charnockite masses in south-east Wainad are found 

 in the Ouchterlony valley and on the Marpanmudi ridge — between 

 Devala and Gudalur on the east and Nellakota and Pandalur on the 

 west. Other smaller exposures occur between Pandalur and 

 Cherambcidi and on the ghat road from Nadugani to Carcoor. 



The charnockite of the Ouchterlony valley is merely the western 



fringe of the Nilgiri mass already described by 

 Ouchterlony valley. & „,•.,, , . ,. 



Mr. Holland : 3 it belongs to the intermediate 



variety and is highly garnetiferous and hornblendic : it is well seen 



on the ghdt road between Naduvatam and Gudalur. 



Much of the Marpanmudi ridge also is composed of charnockite, 



Between Devala and which forms the north-eastern end of the ridge. 

 Pandalur. Further south, where it is crossed by the road 



from Devala to Pandalur, numerous intrusions of acid and inter- 

 mediate forms, but chiefly the latter (PI. 3, fig. 3), occur in the 

 gneiss ; and it would seem that the ridge has a core of charnockite 



1 Mem. Geol. Surv. Ind., Vol. XXVIII, Pt. 2, p. 200. 



2 Mem. Geol. Surv. Ind., Vol. XXVIII, Pt. 2, p. 184. 



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