Geology of Southern India. 191 



i 



ded pebbles, and thus a laterite results. Dr. Clark from 

 his own observations has subdivided the laterite into 

 three classes, the lithomargic, the quartzose, and the 

 detrital. The first is distinguished by the large quan- 

 tity of lithomargic earth present in it, which deprives it 

 almost wholly of tenacity, and gives it a cavernous appear- 

 ance ; the second, by containing numerous rounded pebbles 

 bound together by a clayey cement, occasionally also cavern- 

 ous ; the third, by being merely a congeries of pebbles not 

 firmly aggregated, but yet exhibiting the features of laterite. 

 My own observations induce me to believe that this classi- 

 fication will include nearly the whole of the varieties of 

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

 prominently beyond its range. To Dr. Clark's most in- 

 teresting paper* I would refer those who may wish to 

 see the grounds on which the present theory of the forma- 

 tion of laterite rests, fully illustrated; my sole intention in 

 alluding to it having been to confirm the accuracy of his ob- 

 servations on those localities which we had similar oppor- 

 tunities 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 observing 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 appeared as white earth. Gradually to the 

 depth of 20 feet, the rock approached nearer and nearer 

 to its natural undecomposed state, though even at that depth 

 it was still crumbling. When the excavation ceased at about 



* Madras Journal of Science, December, 1838. 



