1836.] 



between Madras and the Neilgherries. 



it 



"being totally wanting, the roclc then assumes the aspect of basaltic 

 hornblende. This rock, when decomposed, forms a red ferruginous soil. 



About three hundred paces beyond the western extremity of this 

 green-stone, there are some clusters of granitic blocks, some placed, tor- 

 like, one upon the other. On examining one of them, I saw a basaltic 

 dyke, traversing it in an east and west direction, having entered the block 

 from the west ; since its thickness on that side is about five inches, and 

 getting thinner as it advances east, it is finally lost in the substance of 

 the granite (No. 36). 



Guided by induction, I surmised that probably the dyke, be- 

 fore reaching this block, may have traversed those that were seen 

 about fifty paces west of the first cluster, in which case the trap must 

 have a greater thickness than in this, where it appears the dyke spent 

 itself. My conjecture proved correct, since I saw two distinct dykes of 

 basalt traversing, in all their length, two granitic blocks in this second 

 cluster (No. 37), and of greater dimensions than that in the first 

 rock. 



These second masses of granite are not so large, nor so prominent 

 above the soil, as the former ones. They are hardly three feet above 

 the ground, and extend about sixteen in length. In one of them I saw 

 a dyke of very compact basalt, the fracture of which is dull and its 

 texture extremely tough. It has an uniform thickness, all its length, of 

 about three feet, stretching the whole length of the rock. The sur- 

 face of this basalt was divided into rhomboidal pieces. 



In the other block of granite there is a second dyke of smaller di- 

 mensions, and only a few feet from the former. The mass, through 

 which this second dyke passes, seems to have suffered displacement 

 from two splits, which rent it into three pieces, displacing the middle 

 one and causing a fault in the dyke ; the diagram No. 4 represents 

 this fault. Both these dykes converge one foot in ten paces. 



The granite in contact with the basalt appears to have lost its crys- 

 talline structure, and become more friable than the same rock at a dis- 

 tance from the trap. I would have pursued the examination of this 

 dyke further west, but seeing no projecting rocks in that direction, the 

 examination could not be followed up. 



I do not recollect to have ever seen in India the gneiss so well cha- 

 racterised, and its strata so much contorted, as in the locality I am going 

 to point out. North of, and near, the European cavalry barracks, there is 

 an extensive tank, the waters of which are confined, on the east side, 

 by an embankment. About the middle of this rises a huge rock, or 

 rather hillock, about 100 feet above the level of the plain, formed of 

 gneiss, the strata of which, as I said, are contorted in the most striking 

 manner (No. 38). 



The whole mass of this gneiss has the usual convex surface, and 



