DESCRIPTION OF THE CHIEF TYPES. 147 



whilst those taken from the mass of norite will be found to be equally 

 constant in their characters and seldom much above or below 3*09 

 in their specific gravity. These two rocks, therefore, form definite 

 and distinct individuals, the one an acid rock with 75 per cent, 

 of silica, and the other a basic rock with only 53 per cent, of 

 silica. 



But whilst these rock individuals are so well-defined in our 

 Apparently composite type locality, the members of the charnockite 



character of the interme- . t j. ., t ., n ., 



diate group. series found in other parts of the Presidency 



generally present no such constancy of characters. Even in a hand- 

 specimen the composite nature of these a intermediate " rocks is 

 generally very noticeable, the basic and the acid portions being either 

 distributed in irregular patches or arranged in parallel bands. This 

 fact makes it very difficult to place any one specimen in the com- 

 monly employed system of rock classification. 



Whilst the microscope shows an equally marked irregularity in 

 the relative distribution of white and dark elements through the 

 sections of these intermediate members of the charnockite series, one 

 interesting fact presents itself at once, namely, the large number of 

 mineral species represented in each specimen. In fact in nearly all 

 sections of these intermediate rocks all the minerals characteristic of 

 charnockite are found mixed irregularly with all the minerals of the 

 norites. The commonest variety would thus be described as com- 

 posed of quartz, orthoclase, plagioclase, augite, hypersthene, horn, 

 blende, iron ores, apatite and zircon (No. 9*807; section 1534). 

 Such a mineral list would apply to by far the larger number of 

 specimens of the charnockite series picked up in the Madras Presi- 

 dency. 1 



In view of the composite nature of these intermediate varie- 

 ties, one would naturally expect that every gradation between pure 

 norite and pure charnockite ought to occur. This may be true if we 

 take only small specimens into consideration. But by determining 



l The garnetiferous varieties are not taken into this consideration, as they nearly always 

 show signs of having suffered from dynamometamorphism, and I regard the garnet as a 

 secondary constituent. 



( 29 ) 



