286 Memoir on the Geology of the [Oct. 



Before finishing this geological account of part of the Koomlahs, I 

 must ask a question or two, which must have occurred to my readers, 

 while perusing these pages. It regards the decomposition of the 

 basalt, not only in the Koondah, but also in the Neelgherry, groups. We 

 have seen, in the description of the localities where this rock is found, 

 whether as basalt or basaltic hornblende, either intruding as a dyke 

 into rocks, or overlaying them, that, when it arrives at decomposition, 

 it invariably, and in all places, is transformed into yellow ferruginous 

 clay. We have seen and read, also, that, in many parts of the plains 

 of the Carnatic, Northern Circars and Deccan, when the same rock 

 decomposes, an ash coloured, friable clay, or wacke, is formed ; and 

 this decomposition takes place in concentric layers. If the two rocks 

 are analogous, whence this difference when decomposed ? 



Another question seems to me rather of a difficult nature. Since 

 neither sort of basalt, found at present in Central and South India, 

 decomposes into a black soil, what kind of trap, and under what circum- 

 stances different from the present, could this basalt give rise to such a 

 different product ? Is the general opinion of the black soil having 

 resulted from the decomposition of the basalt, one of those that are 

 repeated only because once told ? In my humble opinion, these two 

 questions, and that of the distinction of the different rocks included 

 under the general term of laterite, ought to engage the labours, the re- 

 searches and talents of geologists and chemists in India, to remove 

 many doubts in Indian geognosy.* 



Reviewing all the facts related in the foregoing pages, we may con- 

 clude by a concise comparison between the two groups of mountains. 

 The rocks forming the Koondahs do not contain much hornblende, which 

 deficiency appears to account for the scarcity of the lithomargic earth 

 in this group. The detritus, below the vegetable soil of the Koondahs, 

 has a harsher feel, and is more scabrous than that of the Neelgherries. 

 The second stage of the decomposition in the Koondah rock produces a 

 more crumbly, friable result, than that of the other group. Common 

 granite is the prevailing rock in the Koondahs ; basalt comes next in 

 quantity, and pegmatite last. In the only place where I saw the late- 

 ritic iron ore, I found it precisely similar, in composition, Origin, and 

 geological position, to that of the Neelgherries ; that is, haematitic iron 

 ore, which, in many of the projecting masses, assumes the cavernous, or 

 the caverno-conglomerate structure, flanked and surrounded by, and 

 imbedded in, the primary rocks. In the Neelgherries, granite of four 

 minerals, sienitic granite, common granite, schistous diorite, pegma- 



* We have already an analysis of the black soil of the Southern Mahratta country 

 by Mr. Reid, which makes the composition of the black soil to be very nearly the same 

 as that of basalt.— Edinburgh JVew Philosophical Journal, 



