PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 



219 



very pretty example of this primary breccia is shown near the 

 boat house on the shore of the Ootacamund lake in the Nilgiri hills. 



Fig. 7. — Primary breccia : plan of exposure neat the lake, Ootacamund, 



In a small exposure which has been blasted for the road cutting, 

 some scores of angular fragments of dark-coloured, basic, fine- 

 grained rock are seen to lie in a matrix of lighter-coloured, more 

 quartzose and coarser charnockite. There is a fairly distinct linea r 

 disposition of the acid rock, whilst the basic fragments have their 

 long axes also arranged, stream-fashion, parallel to the general flow 

 (fig. 7). Although at a distance the dark fragments are plainly 

 distinguished from the more acid matrix, close examination increases 

 the difficulty of determining the exact line of separation of one type 

 from the other, whilst microscopic sections show a perfectly 

 gradual, though rapid, transition between the two. This is a 

 constant and an essential feature in these primary eruptive breccias 

 and is shown equally well by the junction-lines between isolated 

 schlieren (autoliths) and the normal rock in which they lie, 



Besides these basic autoliths, which are in general the products 

 of an early stage in the processes of consolida- 

 tion, almost every mass of the charnockite 

 series shows instances of the acid schlieren, 

 which generally take the form of veins, and represent, as in our 



( 101 ) 



Acid 



" contemporane- 

 ous veins." 



