38 GEOLOGY OF TiaCHINOPOLY, &C, [CUAP. III. 



h. Lateritlc DeponHs. — These singular deposits occur, as before men- 

 tioned, very generally on the top of the grits series, without any apparent 

 unconformity ; they were not, therefore, during the progress of the Sur- 

 vey, separated from the Cuddalore series, and appear on the map represented 

 by the same colour. A well marked instance of the apparent passage of the 

 overlying laterite downwards into the mottled grits, occurs at Vellum, in 

 the moat of the old fort, as already noticed by Mr. H. F. Blanford."^ 



The same may also be well seen in the small nullah east of the fort, 

 where the pot holes are excavated in lateritic mottled grits. 



Notwithstanding this intimate association of the laterite with the 

 grits, there appears to be an unconformity when their relation over a 

 larger area is considered, and this we shall endeavour to illustrate after 

 having considered the general lithological character of the deposits. 

 This brown ferruginous deposit, usually called 

 Lithological character. 1^^^^^^^, occurs in two forms over this district ; as 

 a regular aqueous deposit of great extent, or as the effects of decomposi- 

 tion in situ of highly ferruginous rocks. It will be seen that these 

 are virtually the two forms described as " Laterite^-* and *^ Lithomarge" 

 by our colleague Mr. W. T, Blanford in his Notes on the Laterite of 

 Orissa.t This character is well shown in the typical laterite capping 

 the mottled grits both at Capper's Hill near Cuddalore and at Tanjore, 

 though the latter form, as far as we have seen, does not occur as therein 

 described {i. e., underlying the true laterite) . 



The latter variety has been observed, in two or three localities south 

 of the Cauvery, as an assemblage of blocks in the bends of streams, 

 where a small quantity of water almost constantly remains ; and that 

 it is essentially decomposed gneiss in situ, is evident, since the foliation 

 is distinctly visible, as well as the gradual change from a dark reddish- 



* Memoirs of the Geological Survey of India, Vol. IV., Part 1, page 167. 

 t Memoirs of the Geological Survey of India, Vol. I, page 280. 



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