INTRUSIVE ROCKS IN THE ARCH/EAN AREA. 171 



No marked difference exists between the rocks forming the 

 dykes penetrating the Archaean rocks and those seen irrupted into 

 the younger Dharwars, but mere macroscopic inspection is insuffi- 

 cient to settle that question, which must be decided by close micros- 

 copic examination yet to be carried out by some proficient petrogra- 

 phic specialist. 



B. Brecciated Quartz-Runs {Fault Rocks). 



These interesting formations, though not so numerous as the 

 trap dykes, are yet very conspicuous features in many parts of 

 Bellary district, and many of them rise into important ridges and hills, 

 mostly very bare of vegetation on their flanks, while their crests are 

 very blocky, and in some cases formed into precipitous scarps by the 

 action of great master joints. 



In colour they vary from nearly pure white to distinct brown or 

 chocolate colour. The average hue they as- 

 Colour and breccia sum e is pale fawn, but cream colour, very light 

 reddish, greenish, and pale green and brown- 

 mottle are also met with. The brecciated texture is not equally deve- 

 loped throughout, and is sometimes very hard to distinguish, or actu^ 

 ally wanting, while at a little distance it often shows again strongly 

 in the same run. 



The brecciation seems to be a change of colour and translucency 

 in the mass of the rock, in shapes resembling those of included 

 angular lumps rather than a true re-cementation of a once fractured 

 rock, and, but for their angularity, the variations of colour and dia- 

 phaneity would preferably be attributed to lines of fluxion. The 

 greenish brown mottled variety shows the brecciation most strongly. 



They form true dykes, emanating from an ultra-acid magma, and 

 may, like the trap dykes above described, be 

 dykes! qUaftZ rUnS trUG refe rred to a similar set of groups, having each a 

 point of the compass as index of the approxi- 

 mately common strike of certain dykes. 



( <7« ) 



