14.2 GEOLOGY OF TRICIIINOPOLY, &C. [ClLVP. VII. 



account for the northerly inclination of the lowest reach of the Coleroon 

 river_, the sand carried out being constantly spread north by the-up-coast 

 current. 



These currents^ full particulars of which we have not been able to 

 obtain^ are in great measure dependent^ for direction and intensity, on the 

 prevalent monsoon ; but the southerly cm-rent which comes in from the 

 Indian Ocean through Palk^s Straits, seems to be the more powerful, if we 

 may judge by the direction pointing to the north, in which the spits of 

 sand at the mouths of the Mundalaur and Coleroon lie. 



Of the wear and tear of the coast at Porto Novo and l^etween Cudda- 

 At Porto Novo and ^^^'® ^^^^ Pondichery, we have not been able to 

 on ic erry. obtain any reliable or detailed information, but at 



the latter place the sea has made considerable inroads. 



Of changes of physical feature artificially brought about, none is 

 more note-worthy than the formation of local alluvial deposits at any 

 possible level, owing to the extensively practised system of throwing 

 bunds or dams across rivers and streams for the pui'poses of irrigation, — a 

 practice which adds not a little to the difficulties of drawing a well 

 defined alluvial boundary, already adverted to, rendering it necessary 

 always to follow the course of the stream under inspection in an upward 

 direction, in order to meet the first dam by which an artificial alluvion at 

 a higher level may have been formed. 



The difference of level, and consequently of horizontal extension, is 

 Alluvium from arti- ^®^T considerable when large bimds have been 

 raised, as, for instance, on the main western tribu- 

 tary of the lyaur, about 2 miles south-east of the village of Mungalum. 

 Here, a large bund now burst had been thrown across the river, by which, 

 on the upper side, a tract of a dark friable alluvium, like Cotton-soil, 

 several hundred acres in extent had been raised from 4 to 8 feet 

 or more above the surface on the south side of the bund. In the 

 channel of a nullah flowing into the river on the east side, just above 

 ( 364 ) 



