150 HOLLAND: CHARNOCKITE SERIES. 



able to make, I prefer to claim this point as evidence in favour of 

 re£ardin£ the intermediate varieties as the result of the direct con- 

 solidation of the original magma, whilst the acid and basic masses 

 are the result of more perfect differentiation favoured by purely local 

 conditions. 



Two other facts appear also to favour this view of the case :— 



(i) The remarkable resemblance of the charnockite to the norite 

 masses points to the consanguinity of these two extremes. Although 

 one rock is distinctly acid and the other is very basic — one, in fact 

 approaches Bunsen's normal trachytic magma and the other his 

 normal pyroxenic magma in chemical composition — the differences 

 between the two are so well obscured by their remarkable agree- 

 ment in outward appearances that they are often confused with one 

 another in the field. 



(2) The intermediate varieties are the most abundant representa- 

 tives of the charnockite series in South India, whilst large masses of 

 pure charnockite and pure norite are comparatively rare and 

 restricted in their distribution, thus indicating that their separation 

 has been favoured by local and unusual circumustances. 



Although, therefore, the acid and basic extremes so closely 

 resemble Bunsen's t and p magmas in composition, and although the 

 intermediate varieties at first sight appear to be composite in 

 their characters, I think the facts favour the view that the apparent 

 composite character of the intermediate forms is due only to imper- 

 fect differentiation, whilst the occurrences of locally large and distinct 

 masses of charnockite and norite are due to more complete differentia- 

 tion of the original magma. The phenomena presented by these rocks 

 may be regarded therefore as analogous to those of the augite-diorites 

 (diabases) in which the magma may give rise to a microscopic admix- 

 ture of augite-diorite and micropegmatite, or may become under 

 other conditions separated into distinct masses of basic gabbro and 

 acid granophyric rocks. 1 



• C/. Rec. Geol. Surv. Ind., Vol. XXX. (1897), pp. 39 and 40; Quart. Jomn. Geol. Soc. t 

 V«l. LIU <"i89 7 ), p. 416. 



i 32 ) 



