14 



Geology of the Country 



[July 



stone, the rounded marks re-appear and of a deeper colour than that of 

 the rock itself. 



Buchanan took particular notice of these darker spots in the po- 

 lished rock, and attributed them to the crystals of basaltine ( a) (so was 

 augite called at the time he wrote) imbedded in the hornblende ; in 

 which conjecture I think him perfectly correct, as the mineral is augite 

 which gives the described appearance to the rock, and it is seen clearly 

 marked in the above mentioned columns of Hyder's Mausoleum. 



It must be remarked that the veins of the silicious schist, intersecting 

 the gneiss up to its surface, do not penetrate into the overlaying green- 

 stone, showing the posteriority in age of the last mentioned rock. 



In going out of the Fort through the northern sallyport, close to 

 which Tippoo was killed, you come upon the right bank of the Cavery, 

 which washes the walls of the Fort at this place. When I visited Se- 

 ringapatam (March 1834) there being very little water in the river, all 

 the rocks forming its beds were exposed to view, enabling me to judge 

 of their nature. 



The principal rock in it, is gneiss, which appears to extend along the 

 course of this river for a considerable distance ; since I have met with 

 the same rock, jutting above the waters of the same river, at the ferry of 

 Polleapoliam, nearly 100 miles S. E. of Seringapatam. This is one 

 among the many proofs that gneiss is the universal subjacent rock in 

 the table land of Mysore. 



Mounting some of the masses close to the outside sallyport, you 

 stand on blocks of a beautiful porphyry of red colour. This rock cuts 

 the gneiss in the bed of the river in an oblique direction N. E . and 

 S. W. across its whole breadth, and is seen continued on the opposite 

 bank, a little below the northern extremity of AVellesley-bridge. 

 . This porphyry (No. 47) is composed of well defined crystals of red 

 felspar, which occasionally are white, imbedded in a paste of compact 

 felspar of the same colour. Besides these two minerals it contains 

 tourmaline, in numerous needle-shaped crystals distributed through the 

 rock, without having any common direction. The red colour of this 

 porphyritic dyke, through the grey of the gneiss, points it out even 

 from a distance. 



Among the numerous pieces of rock, scattered about the western side 

 of the Fort, are found some of a stratified rock of a porphyritic ap- 

 pearance, composed of red felspar, imbedding pieces of white trans- 

 parent quartz, and having thin veins of beautiful pistachio-coloured 

 actynolite (No. 48). 



J ust below the southern extremity of Wellesley-bridge, along the 

 right bank of the Cavery, I noticed an enormous accumulation of a fria- 

 ble calcarious tufa, somewhat resembling osteocolla, or those calcari- 



(a) Buchanan's Journey, vol. II. page 61, 



