58 



king: nellore portion of the carnatic. 



north dyke of compact porphyritic greenstone, with numerous separate 

 crystals of yellowish-green felspar. Many traces of dykes occur in this 

 neighbourhood, some of which are evidently porphyritic at times and 

 oftener compact and homogeneous. 



An interesting and instructive example of the spheroidal weathering 



of these traps occurs in the Kirkumbadi dyke, 

 Relation betveen sphe- 

 roidal weathering and where it crosses a stream about 4 miles east- 

 join mg. south-east of the fort, in so far as it illustrates the 

 important part taken by jointing in the production of these rounded 

 forms. The trap shows apparent bedding due to a plane of jointing 

 dipping north at 15°, and. it is also cut by north- west-by- west, north- 

 by-east, and. east-west nearly vertical joints. It will be seen from the 

 accompanying portrait, of one of the flat joint surfaces, that a prismatic 



v.. 



<V 



Portrait of a sloping joint-surface of a trap dyke. 



( 166 ) 



