PEIROGRAPHICAL EVIDENCE IN SOUTH INDIA. 221 



(Nos. 9*658 and 0/659). * As usual with schlieren these veins, al- 

 though quite distinctly shown on weathered surfaces, are so closely 

 united with the fine-grained charnockite that the junction line loses 

 its sharpness in microscopic sections. 



The phenomena of contemporaneous or segregation veins is 

 not confined to the non-garnetiferous varieties. In the Nilgiris 

 where the charnockite series are so often garnetiferous, the coarse- 

 grained veins, composed of quartz, felspar and well schillerized 

 hypersthenes, also include fine well crystallized garnets often as large 

 as walnuts. A good instance is shown by a mass of rock exposed 

 behind Oaklands, Ootacamund, and several more were revealed 

 by blasting below Oaklands. 



Schlieren phenomena are thus of two principal kinds — isolated 

 autoliths which are generally more basic than the normal rock they 

 lie in, and contemporaneous veins which are generally more acid 

 than the ordinary rock they traverse. 



Directly connected with the schlieren phenomena, but of a special 

 _ , . kind, is the banding so often, or rather generally, 



Deformation of ' s ' . • 



Sciiiieren and the exhibited by the charnockite series. It is very 



origin of banding. J 



seldom indeed that the bands can be traced 

 for any considerable distance; they are, more strictly speaking, 

 highly distorted, drawn-out lenses which give the weathered surfaces 

 of the rocks a streaky appearance. The slight differences of compo- 

 sition between adjoining streaks give rise to different powers of 

 resistance to the action of atmospheric agents, with the result that 

 the so-called banding is always especially noticeable on weathered 

 surfaces; indeed, it (and the foliation) is often not recognisable at 

 all in fresh hand-specimens. As in the case of the ordinary schlie- 

 ren, it is impossible under the microscope to find a sharp junction 

 line between the dark-coloured and light-coloured bands. I believe 

 the banding to be due merely to distortion of the imperfectly formed 

 schlieren by flow of the magma during the process of consolidation, 



1 I have often noticed in these rocks that the highly quartzose contemporaneous veins also 

 contain considerable quantities of iron-ore which is sometimes titaniferous. A conspicuous 

 instance is exposed near the ■* Castle," Yercaud, Shevaroy Hills. It is not at all uncommon 

 to meet with instances which show that the iron-ore is reserved for the final stages of consoli- 

 dation. 



( IO3 ) 



