DESCRIPTION OF THE CHIEF TYPES. 149 



A very closely agreeing result was obtained by me in the Shevaroy 

 hills : 48 specimens taken from different parts of the mass gave an 

 average specific gravity of 2777. 



From analyses made by my colleague, Dr. T. L. Walker, of 

 Nos. 11*915, 9*785 and 9*791, it seems evident that these intermediate 

 forms are composed of about half norite and half charnockite. 1 A 

 fragment having a specific gravity of 2*787 was found to contain 

 61*40 per cent, silica; one having a specific gravity of 2*8 18 contained 

 58*30 per cent, of silica, whilst a third specimen, having a specific 

 gravity of 2*772, contained 63*77 P er cen t. of silica. A mixture of equal 

 parts of norite and charnockite would have a silica percentage of 64. 



It does not necessarily follow, however, that these intermediate 

 varieties of the charnockite series are actually the result of the mixing 

 together of previously differentiated charnockite and norite magmas. 

 On the contrary, the limited amount of variation which they present 

 would be regarded by some as an argument in favour of regarding 

 the intermediate forms as the result of the consolidation of a magma 

 from which, under special conditions developed locally, charnockite 

 on the one hand and norite on the other have been differentiated. 

 That is to say, the large, well-defined masses of charnockite and 

 norite at St. Thomas' Mount would be regarded by some as the more 

 complete separation of the acid and basic rocks, whose intermixture 

 on a smaller scale gives rise to the apparently composite nature of 

 the members of the charnockite series included in this intermediate 

 group. 



If the intermediate varieties are really the result of the admix- 

 ture of charnockite and norite ; if, that is, they are only contact 

 products, we ought to expect every gradation between the acid and 

 the basic extremes, which, as has already been pointed out, is not the 

 case. Until, therefore, further observations show that the apparent 

 constancy in the composition of these intermediate varieties is only 

 the result of the limited number of determinations which I have been 



1 On account of the specific gravity of the felspars and quartz being in close agreement 

 the density of the rock fragment does not form a safe index of silica percentage. 



( 31 ) 



