1840.] Noies, principally Geological^ on Southern India. 131 



as inorganic, are, in truth, lapidifications of auimal or vegeta- 

 ble matter. 



The formation of the silicious rock is evidently posterior to 

 the gneiss, as we see it, as before mentioned, not only slightly over- 

 lapping and penetrating it, but also penetrating the associated bril- 

 liant hornblende schist. It appears to be however, anterior to the 

 intrusion of the greenstone dykes, in the immediate vicinity, one 

 of which is seen in the fort ditch, slightly overlying it. It is 

 generally of a greenish grey colour, tough, compact, trans- 

 lucent at edges, fracture semiconchoidal, sometimes containing mi- 

 nute shining felspathic crystals. Before the blow-pipe per se it 

 whitens, and melts slightly, and with difficulty at the edges : fuses 

 with carbonate of soda, with effervescence, into a greyish white enamel. 

 An opaque variety, which appears to consist entirely of compact felspar, 

 melts without much difficulty per se into a light grey enamel. 



A dyke of red porphyry (noticed by Dr. Benza), is seen crossing 

 diagonally the bed of the Cauvery, oppositethe sally port, where Tippoo 

 fell covered by the wounded and slain. The direction of the dyke is 

 N. 40 E. It consists of flesh-coloured felspar crystals, imbedded in. 

 a paste of the same mineral, but of a deeper red, having a number of 

 dark green acicular and prismatic crystals, resembling tourmaline, dis- 

 seminated. The gneiss at the junction, presents few indications of al- 

 teration, beyond being rendered more compact and indurated. Large 

 deposits of tufaceous kanker take place on the banks of the river, con- 

 sisting generally of a whitish grey carbonate of lime, with a sponge-like 

 structure, imbedding nodules of a more ancient formation, and a more 

 compact variety ; also bits of felspar, quartz, &c., debris of the subja- 

 cent rock, and roots of grasses. The gneiss is traversed by pegmatitic 

 veins of red felspar, and a light grey translucent quartz, which are 

 themselves penetrated by thin veins of compact felspar, tinged of a faint 

 pistachio green, probably by actynolite, a mineral generally found as- 

 sociated with the pegmatitic veins. The rock composing the green 

 veins, before the blow-pipe, first reddens ; secondly becomes black, and 

 finally melts into a dark emerald. 



Island of Seringapatam. — The island of Seringapatam is formed 

 by the bifurcation of the Cauvery into two streams, which again 

 unite a little to the eastw^ard of their separation. It is about 

 four miles in length, and about one and a half in breadth in 

 the middle. Its surface is convex, sloping to the streams on 

 either side. The fortress is situated on the western extremity, and 

 the suburb of Shahr Ganjam, about the centre of the island. Near this 

 in a cypress garden, stands the handsome mausoleum of Hyder an J 



