246 



Memoir on the Geology of the 



[Oct. 



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

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

 the hrcmatitic iron ore, the conglomerate resembles exactly the pisi- 

 form, or oolitic iron ore, and in some places it is hard enough to be used 

 for architectural purposes. The conglomerate in this state of aggrega- 

 tion 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 ano- 

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

 in enormously thick beds, hereafter to be described. 



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

 stratum of an octireous 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. Jt is this red earth, which, filling up 

 the interstices among the original inequalities of the projecting 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 genera], 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 subsides 

 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 j 

 precisely as we observe the same veins of felspar, in an undecomposed 

 State, traversing the hard rock, which forms the hills. 



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

 tion of two of the rocks, which almost exclusively form the Neelgher- 

 ries ; 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 Koonoor bridge, are of this 

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

 becomes of a soft consistence and earthy texture : the minerals com- 

 posing the rock still retaining their relative position as before. Thus 

 we see in the lithomargic earth, what was hornblende, changed into a 

 red ochrey substance 5 the felspar into a white clay \ the numerous 



