270 Memoir on the Geology of the [Oct. 



substance (Nos. 96 and 97). In the map, these dykes are marked in 

 their proper places. 



At the eastern end of this second hillock many of the masses are of 

 a very coarse, almost porphyritic, species of granite (No. 98). Some of 

 the hills between this locality, and what I call Seven Kairn's hill, are of 

 a quaternary granite, that is felspar, quartz, garnets and mica (No. 98£), 

 and, losing occasionally the garnets, they become common granite 

 (No. 99). 



About half a mile before reaching the ford of the Pykarra on the 

 road from Ootacamund to Neddiwattum, we see a thick quartz-vein, 

 so common in the Neelgherry rocks, but differing from them all, in 

 containing a good deal of talc, the only place where I have found that 

 mineral on these hills (No. 100). The quartz in this vein is white and 

 transparent, occasionally disintegrating into granular (No. 101). 



The same observation made when speaking of the Kaitee dyke, is also 

 applicable to this : the small basaltic veins have a compact, and dull 

 texture, while the body of the dyke itself has a granular structure, 

 somewhat shining (No. 102). 



Basaltic dykes are not rare in those places, which I have had an 

 opportunity of visiting in the plains of India. I have seen them 

 through granite and gneiss in Mysore ; through porphyry, near the 

 euritic hill of Pallicondah ; through hornblende slate, near Mateepol- 

 liam; through porphyry, near Garabunda (Northern Circars), and ill 

 many other places. Are these dykes the fissures through which the 

 enormous mass of trap, overlaying most of the rocks of the peninsula, 

 burst up? and which subsequent events and revolutions having remov- 

 ed, the vents, only, through which it was forced up, remain to be seen ? 



It is a well-ascertained fact that the structure, if not the nature, of 

 rocks in contact with the basaltic dykes, is often greatly changed or 

 modified. I saw nothing of this alteration in the rocks close to the 

 dykes I have been describing. The specimen I send, shews no other 

 change, except a slight diminution of cohesion among the composing 

 minerals, and that not in a very marked manner, nor in every locality. 



Leaving the Cantonment to go to the Avalanche bungalow, on pass- 

 ing the bund of the lake, we must not omit examining the huge masses 

 of hornblende rock close to it. They shew in the clearest manner the 

 progressive stages, through which the rock passes before becoming li- 

 thomargic earth. These masses being split by blasting, we are able to 

 see at once the crystalline nucleus of the rock, together with its state in 

 all the progressive stages which it passes through. I must make a remark 

 regarding the breaking and blasting of the two rocks, the hornblende 

 rock and the hornblende slate, on these hills, particularly the slate that 

 has either very few or no garnets. When the latter is broken pretty 

 deep into its substance, the texture of its crystalline composition is 



