156 Mr Calder on the Geology of India. 



ing rocks exist in this district, and of the black-cotton soil, 

 which is supposed to be produced by the debris of trap. In 

 the neighbourhood of Pondicherry there are beds of compact 

 shelly limestone, and some remarkable siliceous petrifactions, 

 chiefly of the tamarind tree, which have never yet been well de- 

 scribed. The beds of the Cavery, or rather the alluvial depo- 

 sites 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 granite rocks, and plains of ma- 

 rine sand, probably left by the retreat of the sea ; with occasional 

 fresh-water alluvial deposites, and partial beds of iron-clay, and 

 detached masses of the overlying class. In approaching the 

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

 face, and clay-slate and sandstone begin to appear. On the hills 

 behind Pellore 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 is to the observations of Drs Heyne and Voysey that we 

 owe all the information we yet possess of the valleys of the Pen- 

 nar, the Kistna, and the Godavery rivers. This interesting 

 tract of country is not more remarkable as the ancient source 

 of the most valuable productions of the mineral kingdom, being 

 the repository of the Golconda diamonds, than for the extraordi- 

 nary geological features which it presents. The Nella Malla 

 range of mountains, in which the diamond breccia is found, is 

 described, by Dr Voysey, as exhibiting a geological structure 

 that cannot easily be explained by either the Huttonian or Wer- 

 nerian theories ; the different rocks being so mixed together, 

 with regard to order of position, each in its turn being upper- 

 most, that it is difficult to give a name to the formation that 

 will apply in all cases. The clay-slate formation is the name he 

 has adopted, under which are included clay-slate, every variety 

 of slaty-limestone, sandstone, quartz rock, sandstone-breccia, 

 flinty-slate, hornstone-slate, and a tufaceous limestone, contain- 

 ing imbedded in it fragments, round and angular, of all these 

 rocks, all passing into each other by such insensible gradations, 

 as well as by abrupt transitions, that they defy arrangement, 

 and render description useless. It is bounded on all sides, how- 



