180 CRETACEOUS ROCKS OF S. INDIA. [PaRT II. § 2. 



CnArxER XII. — Soils and Superjicial Deposits. 



ThesE;, as occurring in Trichinopoly, South Arcot, and Tanjore, 

 may be classed Geologically under 4 heads, viz. : — 



1. Those formed by the decomposition of the rock in situ. 



2. The fluviatlle alluvium. 



3. Regur. 



4. The sandy soils of sedimentary origin generally more or less 

 ferruginous, and freq^uently containing pisiform nodules, which, when 

 very abundant, are sometimes cemented together into a form of laterite. 



The soils of the first class are of rare occurrence, per se. They are 

 confined to certain tracts on the gneiss, and I have met with but one 

 case, viz., near Thutchuncoorchy, within the area of my own survey, in 

 which it could be decidedly predicated that the surface soil was formed 

 solely of the rock decomposed in situ. The gneiss of the Northern part 

 of Trichinopoly (in the neighbourhood of Volcondahpuram) is so de- 

 composed to a depth of many feet that it is reduced to a sandy gravel, 

 but this is covered with ochre varying in thickness from one foot 

 upwards. In other places gneiss equally decomposed is covered with 

 ferruginous soil, as in the case of the accompanying section. Fig. 16, that 

 Fig. 16. Section of Red Soil in gi^avkl tit near Kydooe, S. Aecot. 



s, Ferruginous sand ; 1 to 2 foct: g\ angular gneiss : g, decomijosecl gneiss. 

 of a small gravel pit near Kydoor, in South Arcot, but in this 

 case, and probably in most others, the red soil was of subsequent 

 formation. 



