1847.] including Notices of Sumatra, the Malay Peninsula, fycr. { 6/9 



induration of laterite clays on exposure to the atmosphere." My 

 opinion therefore was that though proper laterite was nothing more than 

 one of the forms of alteration produced hy plutonic ferruginous gases, — 

 that which, in the arbitrary scale formerly given, I have called the 5th 

 degree, — and that any rock in which a sufficient quantity of clay was 

 present, whether it were purely sedimentary or a decomposed crystal- 

 line or compact rock, or whatever its origin or character in other re- 

 spects was, — would, on being exposed to certain degrees of impregnation 

 by such gases, and under the conditions before adverted to, become 

 laterised. This opinion was abundantly confirmed by later observa. 

 tions, but these also proved that iron alone was capable of producing 

 rocks of a lateritic form. The result therefore was that although pro- 

 per laterite is produced in the mode which I have mentioned, yet that 

 mode is not essential to the formation of a lateritic structure. The 

 only essential thing is the diffusion of iron in ramifications throughout 

 a clayey rock. Get the iron so diffused, and it is of little consequence 

 by what door it was introduced. The only distinctive quality of proper 

 laterite is that it has not merely got the iron, but has been, in various 

 degrees, baked in the process of impregnation, and close examination 

 can always discover traces of this. On the other hand, iron may be 

 introduced by aqueous saturation, and if the soft rocks so saturated have 

 planes of inferior cohesion, as many rocks have, the iron will there 

 accumulate. If the iron solution pervade a homogeneous clayey rock 

 as water does a sponge the segregating or concretionary quality of iron 

 so diffused may gradually draw it into connected nodules or ramifica- 

 tions ; and indeed it is probable that in all cases of volcanic gaseous 

 impregnation of the compact parts of rocks the ferruginous matter 

 remained for a time diffused throughout the rock, and that this segre- 

 gating tendency subsequently superinduced its contraction into ramifi- 

 cations and blotches. Where the gaseous impregnation was weak, it 

 would speedily draw into isolated blotches,— where stronger into isolat- 

 ed concretions, — where strongest, and the heat not too great, into 

 ramifications. Again the iron may be laid up in the heart of a crystal- 

 line rock solidified from a plutonic fluid holding iron, and the essential 

 condition for the production of the laterite structure may be found in 

 decomposed hornblendic, or even black micaceous granites that have 

 not been subjected to any supervening volcanic action, The oxidation 



4 T 



