SCHISTOSE AREAS; ll 



The boundaries between the two rock varieties are in every case 

 Boundaries of areas obscure ) where seen in close proximity, they pass 

 ill defined. { n ^ eacn ther by hardly perceptible gradation, 



as might be expected of rocks of which some parts have from various 

 and as yet unexplained causes undergone more extreme metamorphism 

 than others. Within the granitoid areas, different beds have under- 

 gone varying degrees of metamorphism ; and in some, indeed in many 

 cases, this would appear to hate been influenced by the texture of 

 the rock, for it would seem very probable that the metamorphism 

 has advanced directly in proportion to the coarseness of the original 

 materials of which the old sedimentary rock was composed. This sug- 

 gests the idea that these areas of extra, or apparently extra, metamorphism 

 may have been caused in part, if not entirely, by the peculiar distribution 

 of the coarser sedimentary materials of whish these rocks were originally 

 formed. The peculiarity of such distribution must have depended on 

 the strength and direction of the currents then in action, and on the 

 nature of the materials yielded by the yet older rocks then undergoing 

 denudation. The hypothesis that the granitoid areas coincide with areas 

 of deposition of coarse sediments, helps to explain the difficulties (which 

 would otherwise be hard to deal with) in accounting for the occurrence 

 of small isolated patches of granitoid rocks among the true schists, and 

 similarly of schistose beds within the granitoid areas. 



1. — The Schistose Areas. 



Beginning, as before proposed, in the south-west corner of our area, 

 we find at foot of the Vellakonda mountains, in latitude 15° north, a very 

 wild rugged tract of country made up of mica schists, with a few beds of 

 quartzite. 



To the west this mica schist appears to be underlaid by the newer 



The great Vellakonda Kadapa rocks ; but this appearance is due to a 



great line of fault by which the younger rocks 



have been thrown down, so that they abut against the gneissic beds. 



( 11 ) 



