432 Geological Sketch of the Neilg hemes. [AtJG. 



angle with the horizon, and the basset of which is hardly a foot above 

 the soil. Its dip is west ; its direction nearly N. and S. ; and it is seen 

 continued along the declivity of the hill for some hundred yards. 

 It is traversed by fissures in different directions, giving the pieces a 

 prismatic appearance. Proceeding N. we see in the next hill another 

 and thicker dyke, with precisely the same direction as the former. 



The basalt in this place traverses sienitic granite, and it is seen 

 clearly on the side of the road. The pieces of all shapes, as prisms, 

 cubes, rhombs, are strewed below the newly cut road. Above the 

 road, the projecting masses of sienitic granite are traversed by innu- 

 merable ramifications of the dyke, enclosing between them pieces 

 and masses of the fundamental rock (No. 52). 



The same observation made when speaking of the Kaiti 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-like 

 structure, and somewhat shining (No. 53). 



In some of the Kunda mountains, as that of the Avalache, I also 

 noticed some of these basaltic dykes ; and judging from the numerous 

 rounded blocks and pieces of basalt seen in the bed, and in the 

 banks of the river, which descends from the hills N. of the Avalache, 

 basalt must be very common in that group. 



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 

 erratic hill of Adamanacotta ; through hornblende slate, near Motti- 

 pollium ; through porphyry, near Garabunda (Northern Circars), and 

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

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

 burst up ? and which, subsequent events and revolutions having re- 

 moved, 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. 



The above described are the rocks I had an opportunity of examin- 

 ing on the Neilgherries, having met none of the secondary, and much 

 less of the tertiary class. It would appear from this, that the eleva- 

 tion of this plateau, and probably of the whole chain of the western 



