418 Geological Sketch of the Neilgherries. [Aug. 



a degree of hardness, and approaches a conglomerate ; the small rounded 

 pieces heing agglutinated by a clayey paste, resembling a pudding- 

 stone. This is particularly the case in those localities where it overlies 

 the iron ores, so abundant on these hills. When the subjacent rock 

 is the hematitic iron ore, the conglomerate resembles exactly the pisi- 

 form, or oolitic iron ore, and in some places it is hard enough to b% 

 used for architectural purposes. The conglomerate in this state of 

 aggregation is similar to some varieties of laterite found in the plains 

 of the Carnatic. But this pisiform iron ore is not to be confounded 

 with another rock, which also resembles laterite, and is met on these 

 hills in enormously thick beds, hereafter to be described. 



Below the detritus, in almost all places on the hills, we find a 

 thick stratum of an ochraceous red earth, which occasionally assumes 

 both the appearance and the composition of lithomarge, and for this 

 reason, I shall call it hereafter indiscriminately either lithomargic, or 

 red earth. In some of the lower hills, this stratum is above 40 feet 

 thick, as it is near the bund of the lake. It is this red earth, which, 

 filling up the interstices among the original inequalities of the pro- 

 jecting rocks, has given the hills their rounded appearance, by 

 smoothing all the asperities and irregularities of the original rock ; or, 

 to speak more correctly, the projecting points themselves have been 

 smoothed down by their own decomposition into lithomargic earth. 



In general, this red earth is of a mottled colour, or streaked with 

 different hues of red, yellow, crimson, white, and grey or brown. It 

 feels unctuous to the touch, and crumbles into dust when pressed 

 between the fingers. It does not form a paste with water, but sub- 

 sides to the bottom of the vessel. The different colours of this earth 

 are separate and distinct, having a decided line of demarcation, so as 

 to show that they are produced by the decomposition of separate and 

 distinct minerals. We occasionally find in it thick veins of pure 

 white felspar decomposed into porcelain earth, traversing it in all 

 directions ; precisely as we observe the same veins of felspar, in an un- 

 decomposed state, traversing the hard rock, which forms the hills. 



This red lithomargic mould is evidently the result of the decom- 

 position of two of the rocks, which almost exclusively form the 

 Neilgherries ; viz. the sienitic granite, and the hornblende rock, or 

 primitive greenstone ; of both which we shall speak hereafter. 



It seems that before the rock is transformed into red earth, it 

 passes into a dry friable substance, which sometimes has consistence 

 enough to be cut and used for architectural purposes ; many of the 

 stones used in the construction of the Kunur bridge, are of this 

 nature. The second stage of the decomposition is that, in which it 



