110 HOLLAND: GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



biotite-gneisses of this kind have a wide distribution in South India ; 

 but it is interesting to note that these pebbles agree with the old 

 type of biotite-gneiss, and not with some others in Southern India 

 which are fresher in character and not improbably younger in age. 

 As these are probably the only instances so far of Dharwar pebbles 

 being identified microscopically with the gneisses in an adjoining 

 area, it is altogether too early to consider the value of negative 

 evidence; but one naturally looks forward with some interest to a 

 further examination of these pebbles as a means of confirming or 

 criticising our conclusions as to the relative ages of different groups 

 of crystalline rocks constituting the South Indian Archaean complex. 

 One has, however, to be extremely cautious in generalising from 

 correlations of this kind; for not only have we more than one group 

 of biotite-gneiss among the South Indian crystallines, but it is very 

 likely indeed that the old transition rocks distinguished as Dharwars 

 cover a very wide range of time ; so much so, that the identification 

 of any one Dharwar conglomerate with a particular exposure of 

 gneiss would be wholly insufficient to prove that the whole of the 

 Dharwars are younger than the whole of the rocks forming the 

 gneissic complex. 



(2) Schists of the Salem-Ahtur Valley. 



The thinly foliated schists, which are conspicuous along the 

 Salem-Ahtur road, trend about east and west 



Occurrence, 



past the southern foot of Godamalai, and stretch 

 westward to a point about 7 miles from Salem, where they are cut 

 off rather abruptly by a north-east—south-west " trap-shotten " band 

 of gneiss which probably represents a transverse fault line (p. 38). 

 The rocks referred to by Messrs. King and Foote as " talcose 

 schists u are included in this group; but whilst many of the beds 

 contain hydrated mica and chlorite forming the soft lubricant for the 

 slip-planes in these highly crushed rocks, I have come across no 

 ( 8 ) 



