108 HOLLAND : GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



Gneisses of this kind are exposed near the eastern foot of 

 Occurrence and field the hiu west-south-west of Salem town (No- 

 characters. 1,-892); at the foot of G. T. S. Nagaramalai, 



north-west of Salem (No. 11*902) ; along the northern boundary of the 

 schists (group 2), 1 mile north of Karipatti and 9 miles east-north-east 

 of Salem (11*925) ; south of Salem, in many places near and west of 

 the Namakal road (11*890). The general direction of foliation varies 

 from east— west to east-north-east — west-south-west, with a dip at 

 very high angles generally northwards. The well developed foliation 

 with sometimes even noticeable fissility, stands in striking contrast 

 to the massive, imperfectly foliated, pyroxenic rocks which form 

 large lenticular masses in the older gneiss and weather out into 

 prominent hills. 



None of these rocks can be considered to be quite fresh, the 

 General petrological very evident crushing being accompanied by 

 characters. ^ Q formation of epidote in distinct crystals, 



and numerous spindle-shaped bodies with strong double refraction in 

 the felspars; muscovite and granular colourless sphene are also 

 often present and possibly represent secondary constituents, The 

 list of minerals includes, without regard to origin, quartz, felspar, 

 biotite, muscovite, hornblende, epidote, granular sphene, zircon, 

 opaque iron-ores and apatite. The quartz-felspar and ferro-mag- 

 nesian constituents generally present the usual quantitative pro r 

 portions of the granites. The felspars often show distinct lamellar 

 twinning, but generally of the undecided, ill-defined kind which 

 seems to be so constantly displayed by the Archaean gneisses ; 

 microperthite is not uncommon. 



The mineral composition and the structures are essentially 

 the same in all the exposures of these gneisses; 



Pcculicir f C3.tur©s» 



but the points enumerated so far are common 

 to nearly all crushed biotite-gneisses, and so do not prove in them- 

 selves that the isolated exposures are all members of the same 

 gneiss. But, besides this agreement in common characters, there is 

 ( 6 ) 



