1840.] 



Notes illustrative of the Geology of Southern India. 



517 



oxidising influence of the atmosphere. Tlivoughout nearly the whole 

 of Southern India sienite prevails, and with it laterite is very generally 

 associated. But sienite contains hornblende as one of its principal in- 

 gradients, and the large quantity of oxide of iron this contains pecu- 

 liarly disposes it to yield to atmospheric action. Hence then follows the 

 disintegration of the mass; the felspar furnishes the alumina necessary 

 for the matrix, and the quartz resisting decomposition, and becoming 

 ferruginous from the infdtration of the iron, forms the imbedded 

 pebbles, and thus a laterite results. Dr. Clark from his own ob- 

 servations has subdivided the laterite into three classes, the lithomar- 

 gic, the quartzose, and the detrital. The first is distinguished by the 

 large quantity of lithomargic earth present in it, which deprives it 

 almost wholly of tenacity, and gives it a cavernous appearance ; the 

 second, by containing numerous rounded pebbles bound together by a 

 clayey cement, occasionally also cavernous ; the third, by being mere- 

 ly a congeries of pebbles not firmly aggregated, but yet exhibiting the 

 features of laterite. My own observations induce me to believe that 

 this classification will include nearly the whole of the varieties of 

 this very variable formation, as I have never met with any prominent- 

 ly beyond its range. To Dr. Clark's most interesting paper* I would 

 refer those, who may wish to see the grounds, on which the present 

 theory of the formation of laterite rests, fully illustrated; my sole 

 intention in alluding to it having been to confirm the accuracy of his 

 observations on those localities which we had similar opportunities 

 of examining, and to contribute my limited store of facts for the 

 elucidation of a point which has so long engaged the attention of 

 geologists. 



The depth to which the decomposing influence of the atmosphere 

 will reach, even through the solid rock, is well exhibited when sections 

 are exposed in excavations, an opportunity was afforded me for ob- 

 serving during the sinking of a shaft for a mine in the vicinity of 

 Bangalore. The lithomargic earth commenced at eighteen inches or 

 two feet from the surface, and, as the excavation proceeded, the sides 

 of the shaft exhibited it beautifully variegated in colour. At about 

 15 feet it began to get slightly more tenacious, and the felspar appear- 

 ed as white earth. Gradually to the depth of 20 feet, the rock ap- 

 proached nearer and nearer to its natural undecomposed state, though 

 even at that depth it was still crumbling. When the excavation ceased 

 at about 22 feet, the rock seemed to be pegmatie, as nothing but 

 quartz and felspar were distinguishable, and, judging from its appear- 



. • Madras Journal of Science, December, 1828, 



