PHYSICAL GEOLOGY. y 



very different constitution and durability of the rocks composing each, 

 while erosion must have been considerably facilitated by a system of faults 

 occurring along the boundary ; and had it not been for the very unassail- 

 able constitution of the Velingonda rocks, it is quite possible that this 

 part of the Carnatic might have been nearly as broad as it is in the 

 Madras and North Arcot districts, over which the transition series does 

 not appear to have ever extended much further than the Naggery and 

 Narnaveram hills. As it is, the belt of hard strata withstood the slow 

 wearing of the sea, and it now remains as perhaps the most clearly 

 marked and abrupt step between the lowland and upland on the eastern 

 side of the Peninsula. 



The process of carving out having gone on in both the transition rocks 

 of the step and the gneiss of the plains, it must 

 ! have taken place after the period of the former 



and before the deposition of any later series on the plain below the ghats. 

 The only evidence bearing on the latter point in this area is, that the 

 Jurassic plant shales far out on the plains and near the coast show beds 

 containing what I take to be pebbles of transition or Cuddapah quart- 

 zites. The upper Gondwana beds do, however, lie on the gneiss and close 

 up under the Kambak Droog outlier of the ghats in the Madras district : 

 hence the step and plain must have been in existence at the latest in early 

 mesozoic times. 



As regards minor orographic features, the northern half of the 

 Minor orographic fea- country is marked by many ridges and some roche- 

 tares - moutonnee -like masses of small elevation, such as 



the Narasimhakonda ridge to the west of Nellore and the Buchireddi- 

 palem hill north of the Penner, the southern half having only low and 

 broad hill masses. The ridges which are the most characteristic of the 

 smaller elevations are attributable to the greater frequency of bands of 

 crystalline quartzites among the gneiss series of the middle ground, the 

 intervening softer schistose strata having been worn down to the lower 

 general level of the plains ; and the more rounded and less conspicuous 

 detached hills owe their form to some large masses of trap associated 

 with hornblendie rocks. 



( H7 ) 



