PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 213 



of similar features by the great masses of the charnockite series 

 in South India would be regarded as prima facie evidence in favour 

 of their igneous origin also. The last three considerations, however, 

 are direct tests of origin and include the special phenomena 

 through which our ideas of igneous rocks generally have been 

 derived. 



(a) Form and structure of the great massifs. 



The pyroxene-granulites of Saxony occur in the form of bands 

 or lenses, which, compared with the great 



Size of the great masses. .' . 



masses or the charnockite series in South India, 

 are extremely small. The plateau of the Nilgiris covering over 700 

 square miles is composed almost wholly of this series. The Shevaroy 

 hills, covering 100 square miles, represent another large mass which 

 is extremely uniform in composition throughout, at least quite as 

 uniform as any great igneous ?nassif, say of granite or diorite, is 

 ever found to be. Probably still larger masses occur in the Western 

 Ghats. There is a great difference between masses like these and 

 the small lenses and bands of " pyroxene-granulites " of the better- 

 known occurrences in Europe, where the small bodies more com- 

 monly vary from a few inches to a few yards in thickness. The 

 main "granulite formation " of the Saxon Mittelgebirge is a lens 

 some 31 miles long by 11 broad, or covering about half the area of 

 the Nilgiris ; but instead of being a similar uniform body it is a 

 complex including, besides the different varieties of ordinary <( granu- 

 lite," bands of pyroxene-granulite, biotite-gneiss, cordierite-gneiss, 

 garnet-rock, amphibolite, zobtenite and serpentine. 



These great masses of the charnockite series are either quite 



irregular in shape or show a roughly lenticular 

 Shape. 



form. In the case of small masses the lenticular 



shape can often be made out very distinctly. Several small hills of 



the basic varieties occur around Salem and in the Salem-Ahtur 



valley which show this characteristic. In one of these instances, § 



mile E. N. E. of Karipatti in the Salem-Ahtur valley, a river cuts 



( 95 ) 



