22 CRETACEOUS ROCKS OF S. INDIA. [PaRT I. 



This brief sketch of the general features of the country will suffice 

 to explain the geographical relations of the principal rocks, and we 

 may now proceed to describe more at length the igneous and older sedi- 

 mentary rocks of the district, as regards their classification and mode 

 of occurrence. 



I have already stated that rocks of sedimentary origin occupy the 



^ . . ^ ,, ^ high ground of the Eastern part of our area, the 



Linuts of the two o & i 



f^'^^^^^s- crystalline and semi-crystalline rocks being confined 



to the hills and low country adjacent. The boundary between these 

 tAvo classes nearly coincides with a straight line drawn from a point on 

 the sea-coast, 10 miles North of Pondicherry, to the village of Tripatoor, 

 15 miles North of Trichinopoly,* its continuity being, however, broken 

 by the alluvial plains of the Puniar and Vellaur, which extend beyond 

 the limits of the sedimentary rocks, and thus divide the sedimentary 

 area into three divisions, which we may distinguish by the well known 

 names of Pondicherry, Yerdachellum, and Trichinopoly respectively. 

 To the South of Tripatoor the boundary is more irregular, and after 

 bending some miles to the Eastward, disappears beneath the delta of 

 the Cauvery, To the South of this river sedimentary rocks re-appear 

 to the West of Tanjore, and occupy a large extent of country. But 

 their extension in this direction is unknown, as they have not been 

 traced beyond the limits of the present map. 



I have hitherto spoken of the sedimentary rocks in general terms 

 as a single series, but they comprise, in fact, 



Seclimentaiy rocks. 



several distinct groups of deposits, resting uncon- 



formably on each other, and representing in broken sequence a long 



Geoloo-ic period. As developed in the Trichinopoly area, where they are 



* The Cretaceous rocks, as laid down on Mr. Greenoiigh's Map, are altogether too much' 

 to the West. Their real position is on the area coloured as Gneiss between that assigned 

 to them and the sea-coast. The ferruginous (Cuddalore) sandstones, and a large part of the 

 alluvial fomiation, both of them important features of the Geology of this part of the 

 country, are omitted in this map. 



