6 WALKER: GEOLOGY OF KALAHANDI STATE. 



with that based on field and macroscopic observation. Large quartz 

 grains shew wandering extinction and are frequently surrounded with 

 a border of small grains of quartz (cataclastic structure). The larger 

 felspar crystals of the groundmass are often bent or curved and 

 secondary twinning has been induced by pressure, in some of the 

 plagioclase crystals. All the larger garnet crystals are much shattered, 

 though isotropic when examined in parallel polarised light. From 

 the microscopic slides alone one would therefore be justified in main- 

 taining that since consolidation these rocks have been subjected to 

 great pressure which has induced such secondary characteristics as 

 shattered or bent crystals, granulation, stress phenomena as indicated 

 by wandering extinction in quartz grains and secondary twinning in 

 felspars, a conclusion quite in accord with that reached from field and 

 macroscopic observations. 



In some places where pressure was most intense the whole rock 

 was crushed and granulated resulting in a mylonite which can only be 

 referred to the coarse form by its geographical position. In some 

 cases bands of this mylonite cut the coarse rock almost like dykes 

 (specimens 15167 and 15' 168). This intense crushing is most frequent 

 on the north-eastern borders of the granitoid gneiss. 



Geographically the granitoid gneiss occupies the north- eastern part 

 of the Kalahandi 800-foot plain, and extends south-east into the hills 

 of the 3,000-foot plateau, in a band ten or fifteen miles wide, to 

 Lonjigar and beyond at least as far as Singapore in Vizagapatam 

 district. 



Most of the hills on the plains mide up by these rocks are 

 composed of garnet sillimanite schist described in a later chapter. In 

 the south-eastern extension of the granitoid gneiss into the 3,ooo-foot 

 plateau, the higher points are usually composed of sillimanite garnet 

 schist, while the bases of the hills and the valleys shew outcrops of 

 granitoid gneiss. As to the relationship between these rocks and the 

 charnockites, there is very little positive information, but two main 

 facts, (1) both being undoubtedly of igneous origin, and (2) the 

 presence in both of a pyroxene with the pleochroism of hypersthene, 

 should be borne in mind. It may be that they are derived from a 

 common magma which differentiated into more basic— charnockite— 



