THE GNEISSIC SERIES. 21 



recognisable), or again very fine-grained and then sharp and smooth in 

 its outlines and surfaces. The quartz is of white colour, in two forms, 

 dull and amorphous or glassy, while the felspar is generally of pale 

 flesh-colour, or occasionally of a dull white, the rock itself being essentially 

 of a pale reddish colour. It is very hard and compact, and requires 

 most laborious chiselling when being dressed, but is easily hewn into 

 large blocks by wedge splitting, a practice which is or was very 

 common, for wherever there is a mound or hill of this gneiss, 

 there are often old trenches whose sides show the holes originally 

 cut for the reception of the large iron wedges. It is this variety of 

 gneiss which oftenest gives the rocke-moutonnee-like form to many 

 of the hills, and in this feature 1 it answers somewhat to the " dome 

 gneiss " of the northern area of the Bengal gneiss. Foliation is very 

 faint, but what can be made out in the Venkatagiri country is o-ener- 

 ally north-south in its direction and vertical : here also signs of bed- 

 ding are rare. Perhaps along the southern edge of the field, on the 

 right bank of the Swarnamukhi, the rock would be considered more 

 like gneiss, for here there is occasionally distinct and wavy foliation. 



Jointing is developed at wide intervals in the rock generally in 

 north-north-east to south-south-west and east-by-north to west-by-south 

 directions, the latter having the lower dip 60° — 75° southward. 



Granitic and trappean intrusions are not common, or rather they 



Granitic and trappean are not often visible here owing to the area being 

 intrusions. j n p art a r ' ver va ]i ev an( j mnc [ l COV ered up by 



superficial deposits. Dykes of compact and porphyritie greenstone are 

 however often visible in the Swarnamukhi valley, particularly on the left 

 bank at Kirkambadi and thence eastward and westward on the rio-ht 

 side of the valley. Eastward of Pridi (on the railway just south of 

 map) there are again many east-west dykes of greenstone, this locality 

 being just on the eastern edge of a very remarkable and extensive 

 development of trappean intrusions in that part of North Arcot. In 

 the banks of the large nala about 3 or 4 miles eastward of Kirkam- 



1 Manual of the Geology of India, p. 20. 



( 129 ) 



