IN SOUTHERN INDIA. 



291 



The principal geological features of this large tract of 

 country are (1) the coast alluvium with its fringe of blown 

 sands on the east, and on the west the associated alluvia of 

 the Panar, Gruddalam, Vellar, and Cauvery rivers, the latter 

 of which forms a large delta, (2) the western boundary of the 

 alluvium which is generally underlaid by lateritic formations 

 underlaid again in their turn, (3) by a group of sand-stones 

 and grits of tertiary age to which Mr. H. F. Blanford gave 

 the name of Cuddalore sandstones. These in their turn, (4) 

 rest upon the cretaceous rocks which, as already mentioned, 

 rest either upon, (5), a thin series of plant-bearing shales of 

 Jurassic age, or else, (6), direct on the gneissic rocks which 

 occupy considerably more than half the area of sheet 79. 



The specially interesting features of the coast alluvium are 

 the evidences of considerable elevation of the land within 

 recent times and of considerable encroachment of the sea 

 which is now in progress all along the coast. In connection 

 with the Cuddalore sandstones the most remarkable fact is the 

 occurrence of silicified trunks of trees in the grit beds at 

 Trivicary (Tiruvakkarei) some miles west of Pondicherry. 

 Much interest attaches to the jurassic plant beds underlying 

 the cretaceous rocks, but it will be more convenient to refer to 

 them again when treating of other and more important 

 outcrops of the same age near Madras. 



The gneiss country offers many points of interest both 

 scientifically and practically, but only the more prominent 

 can be touched upon here. The most striking feature is 

 unquestionably the presence in large numbers of beds of 

 magnetic iron which, from their well-marked peculiarities, are 

 much more easily traced over great distances, and thanks to 

 which it was possible in some parts of the country to make 

 out the stratigraphy of the series in a way in which it could not 

 possibly have been done if only the ordinary hornblendic or 

 micaceous forms of gneiss had occurred. This is particularly the 



