GEOLOGY OF INDIA. 9 



or concentric lamellar concretions, and in apparently stratified masses, 

 forming low detached hills, the strata of which dip at an angle of abont 

 45° to the S. W.* Partial deposits, of the overlying rocks exist in this 

 district, and of the black cotton soil, supposed to be produced by the de- 

 composition of trap rocks. In the neighbourhood of Pondicherry, there 

 are beds of compact shelli/ lime-stone, and some remarkable siliceous petri- 

 factions, chiefly of the tamarind-tree, which have never been well described. 

 The bed of the Caveri, or rather the alluvial deposits in the vicinity of 

 Trichinopoly, produce a variety of gems corresponding to those of Ceylon : 

 in general, however, the surface of the level country, as far north as the 

 Pennar river, seems to consist of the debris of granitic rocks, and 

 plains of marine sand, probably left by the retreat of the sea, with 

 occasional alluvial deposits, and partial beds of iron-clay, and detach- 

 ed masses of other rocks of the overlying class. In approaching the 

 Pennar river, the iron-clay formation expands over a larger surface, and 

 clay-slate and sand-stone begin to appear. In the hills behind Nellore^ 

 are found specimens of a very rich copper ore, yielding from fifty to sixty 

 per cent, of pure metal, according to Dr. Heyne, besides argentiferous 

 galena. 



It 



* These appearances, hitherto considered foreign to the nature or aspect of granite rocks in 

 other parts of the globe, miglit be deemed questionable here, did they not coincide with similar 

 appearances throughout the peninsula, and remarkably so with those of the Ceylon granites as thus 

 described by Dr. Davy. " In structure, the granitic varieties most commonly exhibit an appearance 

 " of stratification. It is not easy to decide with certainty whether this appearance is to be attri- 

 " buted to the mass being composed of strata, or of large laminae or layers. I must confess I am 

 " more disposed to adopt the latter notion. I have found some great masses of rock decidedly of 

 " this structure ; — masses almost insulated, quite bare, several hundred feet high, in which the 

 " same layer might be observed spreading over the rock, like the coat of an onion ; — and which, if 

 " only partially exposed, might be considered a strong instance of stratification ; — and, if examined 

 " in different places, on the top and at each side, might be considered an extraordinary instance of 

 '* the dip of the strata in opposite directions. With this hypothesis of the structure of the rocks, 

 " the appearance of stratification in all the granitic varieties may be easily reconciled." 



