156 Records of the Geological Survey of India. [vol. xii. 
7iitigrifoli'u/ni) is also cultivated to a very great extent all over the laterit' j area, 
but some of these trees are common on the gneissic area. 
Of minute plants one of the most characteristic, which occurs in immense 
numbers in low Ijdng damp sandy flats, is a small Drosera or sundew, though 
elsewhere a very rare plant in the low country. A zoological peculiarity of the 
lateritic sandy region is the great frequency of a large SpongiUa, or freshwater 
sponge, in many of the shallow rain-fed irrigation tanks, hlowhere else have 
I noticed this sponge to be common or to attain to anything like the size it does 
in this quarter. 
Alluvial Formations .—The alluvial deposits coming within the scope of this 
l^aper are of small extent, as only a small section of the south-western corner of 
the Cauvery delta was examined, and the alluvia of the various small rivers 
traversing our area are very limited. 
The western part of the Cauvery alluvium is formed of a black clay, appa¬ 
rently a true regur, becoming gradually sandy as the sea is approached. Ko 
section penetrating through this black clay was seen mthin the limits of sheet 80, 
but a fresh excavation for the foundations of a new sluice branching off from the 
Pamani-4r just within the area of sheet f’9, on the road from Manargudi to 
Neddamangalam railway station, revealed a bed of very stiff blue clay full of 
small kankar (gravelly tufa). 
The alhivia of the smaller livers is generally a whitish mixture of sandy 
clay with lateritic pellets and small debris of quartz and gneiss. The flats are 
often slightly swampy, or barren, and unproductive from the saline matters 
enclosed. An exception to the whitish alluvium is offered by the alluvium of 
the Manimut-ar below Tripatui’, where low cliffs of reddish loam form the river 
banks for a couple of miles or more above Neddarakotai. The alluvium of the 
Vaigai appears to be generally sandy, but no sections were seen, and from cen¬ 
turies of wet cultivation the whole surface must be looked upon as really 
“ made ground.” 
The coast alluvium near Adrampatam is generally clayey near the surface 
and edged with a narrow strip of ill-defined sandhills. No sections of the coast 
alluvium were seen. 
Soils .—The soils depend almost everywhere on the underlying rocks for their 
character. Eed and reddish sandy soils abound. Black soil is not at all com¬ 
mon. It occurs largely only over the western side of the Cauvery delta, and 
under a few important irrigation tanks where it must be regarded as of artificial 
origin. Whitish clayey soil very similar to the pale alluvia of the large streams 
is met with in many valleys of the eastern lateritic areas, and, as above men¬ 
tioned, often forms small treacherous quicksands most disagreeable to the rider. 
Where the conglomeratic laterite occurs two forms of sod. prevail, both of 
them hard clayey sands, the one of bright red, the other of pale yellow (bath- 
brick) colors—often approaching in texture to true sandstones. Many large 
spreads of these occur covered with low scrub of Boclonma viscosa and a few 
dwarf mimosEe and other thorny bushes, e. g., on the high ground to the south 
of Gandarakotai in the Tanjore patch, and again on the high ground north¬ 
east-by-east of Alangudi in the Pudukotai patch. The surface of the soils is 
