44 GEOLOGY or TRICHINOPOLY; &C. [ClIAP. III. 



Rock/^ at Tricliiiiopoly. Three small isolated patches at the villages 

 of Rangumputty^ Eatchacamalputty, Goondoor^ and Poodoor indi- 

 cate a former connexion with the eastern or main pat<ih. Westward 

 of the Coreyaur^ (an affluent of the Cauvery at Trichinopoly,) no lateritic 

 conglomerate appears, exceptmg a very small patch only a few yards 

 square at the western extremity of the southern tank at Baker^s Choultry 

 (Rotikaranchuttrum) . 



There can be little doubt that this laterite south-east of Trichino- 

 poly is a tn^ie sedimentary formation like the laterite of Tan j ore and 

 Vellum, and is not to be ranked among the sub-aerial j>seudo- 

 laterites, such as those of the Nilgiris and Shevaroys^ which have resulted 

 solely from the oxidization of weathered ferruginous materials, and main- 

 ly from the hornblendic or amphibolitic rocks of those mountains. 

 Should it be imagined, however, that it is a laterite formed by the de- 

 composition of the gneiss rock i>i situ, there are several valid objections 

 to that supposition, and it will most likely be established, when the 

 country around Poodoocottah (in Tondiman^s territory) shall have been 

 surveyed, to be of the same age as, and a continuation of, the laterite 

 Iving on the mottled grits of Vellum and Tanjore. 



The objections to the application of the decomposition in situ 

 theory, in this case, are principally three : — 



1st. — The underlying gneiss rock when exposed is almost invariably 

 quite fresh and undecomposed in contact with the bed of lateritic con- 

 glomerate; whereas, had decomposition in situ been the cause of the 

 latter formation, there would be evidences of such decomposition in the 

 shape of only partially decomposed portions of gneiss at the line of 

 junction. Such, however, is scarcely ever the case over the whole laterite 

 areas westward from the Konjumputty valley. 



2nd. — The formation of such masses of ferruginous rock could only 

 take place by the decomposition of a rock containing an abundance of 

 iron among its constituents, as, for instance, \cvj hornblendic gneiss, etc. ; 



(2GG ) 



