40 GEOLOGY OF TRICHIXOPOLY^ &C. [ChAP. III. 



Where the deposit is thoroughly exposed on all sides, as at a spot 

 7 miles south-south-east of Trichinopoly, where 

 chfraclen '"^ ^"^^"" ^ ^^^^^^ ^^« cut through the laterite, as well as 

 the gneiss on which it rests, the vertical section 

 shows a regular pisiform structure throughout, accompanied by tubiform 

 cavities, though of a more ferruginous character towards the upper 

 surface. It is mainly on the sides and under-surfaces of blocks that the 

 tubiform character is seen; the upper showing them less frequently. 

 Of the cause of these tubiform vermicular cavities, no altogether satis- 

 factory solution has yet been offered, nor did the phenomena observed 

 by us suggest any as yet unobserved cause to which to attribute them.''*' 



South of Vellum there is a section in a stream showing, at a depth 

 of 3 or 4 feet below the surface, veiy cellular laterite permeated by 



* Since the examination of laterite in the country described above, more extended 

 observations of that rock in the Nellore district, where it is largely quarried, have been 

 made, and these, taken in connection with what we have seen of laterite in South India, 

 appear to indicate a reason for the occurrence of the tubiform cavities. This peculiar struc- 

 ture is developed only, or perhaps more correctly to the greatest extent, in superficial laterite, — 

 that ferruo'inous, sandy, or concretionary clay which has become, through exposure to atmosphe- 

 ric influences, what is peculiarly called laterite. This clay is very varied in its composition, 

 bein"' streaked and mottled with little strings and nests of differently constituted clays. In 

 the Nellore quarries especially, as well as in the Tanjore and Trichinopoly laterite, I have 

 observed that the freshly cut blocks of rock are not nearly so visicular as those which have 

 lain about on the ground for some considerable time : and the quarries are excavated in 

 successive flat horizontal terraces until a depth is attained at which the rock is scarcely at 

 all porous and too clayey for the required purpose, when the place is left to be again resorted 

 to after the newly exposed rock may have assumed a harder constitution. 



Taking the above facts into consideration, it would appear, then, that evaporation, con- 

 traction, and the varied constitution of the rocks, in so far as it consists of nests and strings 

 of more or less ferruginous, or sandy, or aluminous materials^ may have tended to the forma- 

 . tion of hollows and vermicular cavities whence etisily disintegrated matter may have been 

 removed by percolation of water, — ^Y• K, 



( 262 ) ■ 



