1835.] Geological Sketch of the Neilgherries. 437 



variety of gneiss abounding with albite, the continuation of which 

 is seen N. and near Pundy. 



These porphyric hills, therefore, may be considered as the out -goings 

 of an enormous dyke of porphyry, which burst through the hills, hav- 

 ing the same direction with them, that is N. E, to S. W. ; their 

 appearance is that of huge masses of a black looking unstratified rock ; 

 in many places completely divested of any sort of vegetation, particu- 

 larly in those hillocks, which like the one called Chittakunda, rise 

 in abrupt, vertical cliffs, which seen within a moderate distance might 

 be taken for basaltic rock. 



The porphyry exfoliates in thick concentric laminae, the more 

 depending portions of which falling off, leave the upper in immense 

 tabular masses, or cubic blocks, perched on the upper part, and some- 

 times on the declivity of the hill : this porphyry has a good deal of 

 hornblende in its composition, sometimes so much, as to become horn- 

 blende prophyry. 



In more than one of these masses of porphyry, I remarked thick 

 veins or nests of a granitic rock, or rather gneiss, with pieces of 

 sienitic granite imbedded in it. The crystals of felspar in this por- 

 phvry are well defined, many of them two or three inches long, and 

 of a foliated structure. This porphyry seems, as I have said, to 

 extend as far as near the sea-shore at Pundy. Some huge masses of 

 it are seen jutting through the soil about a mile north of the village 

 of Carvera, flanked by the gneiss containing ablite and garnets. 



I have put up many specimens of laterite from different localities, 

 by which may be clearly perceived the distinction between the original 

 rock and the conglomerate bearing the same name; but which evi- 

 dently arises from the conglutination of the detritus of the former. 

 This appears to be the case with the laterite in some places of the 

 plains of the Carnatic. 



The specimens (No. XXIII. to XXVI.) are from the hillocks, on 

 which the fort of Puddayaram (near Samalcottah) is built. The 

 position of the visible rocks in this place is the following : the 

 ferruginous sandstone is the lowermost, and has a great degree of 

 compactness, so as to fit it for architectural purposes, in which it 

 seems to be largely employed. It is evidently stratified, the strata 

 being nearly horizontal ; the quartz particles are agglutinated by a 

 ferruginous cement. 



The sandstone, nearly in the whole extent of the hillock, supports 



a lithomarge of a whitish or flesh colour, sometimes having a bluish 



tint. The stratum of this earth is not very thick, and in many 



places, it is overlaid by a purple-red, compact, slaty hsematitic iron ore, 



3 L 



