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Notes illustrative of the Geology of Southern India. 



[ArRiii 



the laterite was the result of the aggregation of, the decomposed con- 

 stituents of the rocks which it usually overlies, was forced on my mind 

 by observations in several of those identical localities whence I find 

 Dr. Clark has gathered a considerable portion of his information | and 

 it was with pleasure that I found a conclusion, based on limited data, 

 supported by an examination so extensive and so ably conducted as that 

 of Dr. Clark. In the vicinity of Bangalore, where both the lateritic 

 and litbomargic formations are largely developed, an opportunity was 

 afforded me of studying them in detail. Many of tbe protruding masses 

 of sienitic granite and gneiss, which are the prevailing rocks, exhibit 

 nature as it were in the simultaneous acts of decomposition and aggre- 

 gation. Their surfaces are usually covered with a coating of dark 

 coloured and Aveathered matter resulting from the felspar or hornblende, 

 studded with small undecomposed and ferruginous pebbles of quartz, 

 while round their bases the laterite forms a regular talus of variable 

 size. It is usually of the compact quarlzose and distinctly marked 

 variety: sometimes however it assumes the cavernous or vesicular 

 structure, the dark red matrix exhibiting a series of small cavities. 

 One example was of peculiar interest, for, in consequence of the exist- 

 ence of a basin-shaped hollow in the side of the rock, the materials 

 necessary for the formation of the laterite had collected there and a 

 small nest of it formed. The circumstances of the case excluded from 

 consideration both the aqueous and igneoua causes, whose aid has 

 usually been called in to explain tbe origin of laterite, and the clear 

 evidence of its being due to the slow but certain operation of atmos- 

 pheric agents amounted to demonstration. The bewildering effect of a 

 multiplicity of contending opinions, gives to such simple yet well mark- 

 ed observations on the point at issue, additional value and interest. 



As the resultants of decomposition, it is natural lo expect that the 

 character and composition of both laterite and lithomarge will be very 

 variable, since the constitution of the parent rocks will necessarily in- 

 fluence that of the derivatives. Hence it has been found by observation 

 that those rocks in which the proportion of quartz to felspar and horn- 

 blende is considerable, decompose into laterite, while those in which 

 quartz is less abundant usually exhibit the litbomargic character. The 

 same specimen sometimes presents both of these formations, the one 

 passing imperceptibly into the other, and clearly establishing the posi- 

 tion that they are effects of the same cause modified by concomitant 

 circumstances. These circumstances are externally the variations of 

 heat and moisture, and internally the greater or less degree in which 

 the constituents of the rocks predispose them to be affected by the 



