Chap. II.] alluvial DErosrrs and bi.ou-n sands. 27 



dering" the exact detei-mination and laying down of the same a matter 



of great difficulty and sometimes an . impossi- 

 obscures all boundaries. o ^ 



bilitj. In the case of the tributary streams from 

 the north, this difficulty occurred pretty frequently, for bunds have been 

 built across the streams at various points, so as to form tanks, and these, 

 "vvith the artificial channels running" parallel to the river, have assisted in 

 the formation of alluvial flats extending often a mile or more beyond the 

 true boundary. 



Nature of the Alluvial Soils, — Two principal varieties of soil occupy 

 by far the greater part of the surface of the delta. 



Soils of alluvium. 



and are very nearly equal in extent of develop- 

 ment ; 1st, dark humus ; 3nd, pale yellow sandy soil. 



The dark humus occurs chiefly above and about the head of the 

 delta, the sea-board and adjacent country having a decidedly sandy char- 

 acter ; many .parts indeed, if not u'rigated, Avould speedily become a per- 

 fect desert. The humus is, where dry, not unlike cotton soil in appear- 

 ance, but less friable ; when wet, it has considerable plasticity, without, 

 however, (in general) assuming a clayey character. The sandy districts, 

 when well irrigated, are by no means unfertile, though vegetation has 

 not quite the same unbounded luxuriance as on the dark soil. Clayey beds 

 ai-e very rare, the best marked is a hard ferruginous bed of clay exposed by 

 the surf on the north side of the mouth of the Cauvery at Cauvery- 

 patam (Cauvery-putnam) . This bed is only exposed to a limited extent. 



Bloicn sands of the Cauvery delta. — Of these, there is no great deve- 

 lopment between Devicottah and Negapatam^ generally nothing more 

 than a narrow ridge forming the inner edge of the beach. The excep- 

 tional cases are— 



Igtly. — Isolated lenticular patches of no very great extent, one 

 on either side of the road, commencing about a mile south of 

 Anagarachuttrum (at which place the Coleroon is crossed by a bridge 



(219) 



