1836.] 



Laterite, or Iron Clay* 



105 



case, we may easily suppose how a rock, of the character of laterite, 

 might he formed from the entire decomposition of basalt, containing, 

 in that peculiar locality, more hornblende, and consequently a larger 

 proportion of iron, than usual, or perhaps intermixed with some of the 

 numerous iron ores so abundant over India. Bergman states the 

 average proportion of iron contained in basalts to be 25 per cent, so, 

 perhaps, that quantity alone might be sufficient, to afford the ferrugi- 

 nous components of Voysey's iron clay. Dr. Benza, in his admirable 

 Geological Sketch of the Neilgherries, states the red lithomargic earth 

 to result from the decomposition in situ of the sientic granite and the 

 hornblende rock, and, he says, " we see in the lithomargic earth what 

 was hornblende changed into a red ochrey substance j the felspar into a 

 white clay ; the numerous garnets into a crimson coloured clay ; the 

 quartz alone remaining unaltered and undisintegrated." Now it ap- 

 pears to me, that {mutatis mutandis, the rocks differing in the two 

 localities), here are all the elements for the composition of laterite 

 such as Voysey encountered in the Hyderabad country, the red ochrey 

 substance, aided, most likely, by ores of iron disintegrated and inter- 

 mixed, forming the principal component. 



But we must not lay it down as an established fact in geology, from 

 the evidence hitherto brought before us, that laterite is a " contempo- 

 raneous rock associating with trap." Mr. Calder may have indepen- 

 dent proof to offer, but it is necessary to state it. It is, apparently, on 

 his authority that Mr. Conybeare describes the laterite in the terms 

 I have quoted at the commencement of this essay ; for the latter gen- 

 tleman, in his address, cites Mr. Calder's paper as the ablest summary 

 we have of Indian geology. 



In the passage I have quoted from Voysey, the mineral at Beder is 

 said closely to resemble that of the Red Hills near Madras. I hope 

 satisfactorily to prove, by and bye, that, at the latter place, it is a con- 

 glomerate rock, evidently of detrital origin. The Nellore laterite, of 

 which 1 have a hand specimen given me by Mr. Malcolmson, is composed 

 of innumerable minute pebbles of quartz, rarely larger than half the 

 size of a pea, sometimes pellucid, generally much rounded ; together 

 with red and yellow ochraceous earths. The specimen I have, is not so 

 full of cells and sinuosities, as laterite is usually observed to be. A 

 specimen of white clay, said by Mr. Malcolmson to underlay the late- 

 rite at Nellore, is also in my possession. This, most probably, however, 

 forms a very partial substratum, being, perhaps, nothing but a decom- 

 posed felspar vein, in the underlaying granitic rocks.* 



* Since writing the above, I have been favoured by Colonel Cullen, 

 Commissary General of the Madras Army, with the perusal of a letter 



