DESCRIPTION OF THE CHIEF TYPES. 153 



through an elaeolite-syenite in the Coimbatore District, its presence 

 therefore in these rocks may be quite consistent with the evidences 

 which indicate their igneous origin. It will be interesting to see if 

 the veins of graphite which occur in Ceylon and Travancore pass 

 through members of the charnockite series. 



[Since the above was written an interesting memoir (" Beitrag zur Kenntniss 

 der Gesteine und Graphitvorkomnisse Ceylons ") has been published by Max 

 Diersche, giving an account of granulites and pyroxene-granulites collected in 

 Ceylon. The author failed to find graphite as microscopic constituents of these 

 rocks, although the graphite veins occur near the granulites and include fragments 

 ©f both the ordinary and the pyroxene-granulite (Jahrb. der k.-k. geol. Reichs, 

 XLVIII (1898), 241, 257, 279, 284 and 286).] 



(3) Basic Division. 



Associated with the charnockite masses at St. Thomas' Mount 



and Pallavaram, and forming distinct masses uniform in com* 



position over large areas, are rocks which in mineral composition 



agree very closely with the rocks known gener- 



Use of the term norite. , . ., . . ^ 



ally as norites. Norite is one 01 the many 

 names which, on account of the changes introduced into petrographi- 

 cal nomenclature as a result of the use of the microscope, has under- 

 gone a variation in meaning since its first use in 1838 by Esmark. 

 In describing these rocks the word norite is used for rocks composed 

 essentially of plagioclase and rhombic pyroxene, the meaning which 

 has generally been given to it in petrographical literature since the 

 limitations proposed by Rosenbusch in 1877. Under the names 

 augite-norite and olivine-norite I have previously described many of 

 the dyke rocks which cut through the charnockite series in South 

 India. 1 Although there is not the slightest difficulty in distinguish- 

 ing between these and the members of the charnockite series which 

 have a corresponding mineralogical composition it seems inadvisable 

 to complicate the already confusing petrographical nomenclature by 

 proposing modifications in name to indicate the other differences which 

 are evident in hand specimen as well as under the microscope. The 



1 Rec. Geol. Surv. Ind., Vol. XXX (1897), p, 16, 



D2 ( 35 ) 



