]2 GEOLOGY OF TRICHINOPOLY, &C. [ClIAP. I. 



The general geological conformation of the countiy is very simple. 

 The greater part of the area is occupied by meta- 

 ° ' morphic rocks belonging to the gneiss family ; 



resting on these are three great groups of sedimentary rocks belong- 

 ing to different geological periods, and overling each other in regular 

 succession from west to east. The first of these great groups belongs 

 to the Cretaceous era. Resting upon these cretaceous rocks are, secondly, 

 a group of rocks whose exact age has not yet been determined, owing 

 to the absence of organic remains, but which are provisionally dis- 

 tinguished as the post-cretaceous rocks. Resting on these again are 

 the beds of the fluvio -marine alluvium of the coast and river deltas. 

 The sedimentary formations form great bands running in a north-east 

 by north to south-west by south direction, and widening generally as 

 they extend southward. 



The general relative position of the geological formations to the 

 descent of the country towards the Bay of Bengal is exhibited in 

 the accompanying section from Salem to Negapatam (Plate II), on 

 the scale of four miles to the inch. Of these great groups of rocks, those 

 belonging to the Cretaceous system have already been described very 

 fully by Mr. Henry F. Blanford in the preceding part of the present 

 volume of the Memoirs of the Survey. In that report a description is also 

 given of a small tract of metamorphic rocks lying between the cretaceous 

 area in Trichinopoly district and the alluvium of the Cauvery delta. 



Mr. Blanford^s report treats also at some length of some parts of 

 the post-cretaceous formations. With these exceptions, the present report 

 wiU describe the whole area included in sheet 79 of the Indian Atlas. 



As the mountain groups of the country embraced in this report are 



of great interest geologically and topographically. 

 Mountain groups. 



and are very frequently referred to in these pages, 



a brief description of the leading features of each wiU be foimd useful and 

 ( 234 ) 



