544 Malcolm Maclaren — The Origin of certain Laterites. 



generated in areas of deficient moisture. Its soil is crowded with 

 'kankar' — small irregular concretions of carbonate of lime — which 

 may, in tlie stream beds, form a calcareous tufa, while the rivulets 

 that do run in the dry season through the black cotton soil are 

 bitter with alkaline salts. Tliese are exactly the products removed 

 in regions of abundant rainfall. 



Except when the humidity is great, the laterite crust supports 

 a very scanty vegetation, and, even in humid areas, it is probable 

 that much of the carbon dioxide essential for carbonation is derived 

 from the decaying humus on the hill-sides above the laterite basins. 

 The older laterites on the hill-tops are devoid both of vegetation 

 and of soil. 



The chemical aspect of the Indian aluminous and ferruginous 

 laterites has been well developed by the analyses of Dr. Warth.' 



Some little attention has been paid by the present writer to the 

 nature of the minerals present. The bauxite, especially selected for 

 examination since it promised to yield the most conclusive results, 

 was one from Katni, Jabalpur district, which had been analysed in 

 the Geological Survey Laboratory,^ as follows : — 



AI3O3 65-48 



FejOs 3-77 



H2O 18-32 



Moisture 1-06 



SiOa 0-38 



TiO., 11-61 



MffO trace 



100-62 



The molecular ratio of AI2O3 to H2 is lower (1 : 1-58) in this 

 specimen than in any other analysed, and for this reason it was 

 expected to show more clearly than any other the presence of 

 diaspore, should the assumption of Warth and others, viz. that 

 bauxite is a mixture of gibbsite and diaspore, be correct. Neglecting, 

 as Warth has done, and with sufficient reason,^ the consideration of 

 the water due to possible limonite in the specimen, the foregoing 

 ratio of 1 : 1-58 represents 29 per cent, gibbsite and 71 per cent, 

 diaspore. It was, therefore, considered probable that a separation 

 might be effected with a heavy solution, the specific gravity of 

 gibbsite being 2-4 and of diaspore 34. Carefully and finely ground 

 bauxite was treated with Sonstadt solution of 3-07 s.g. All except 

 very fine black grains (ilmenite) floated. The specific gravity of 

 the specimen as a whole was taken, and gave incidentally an 

 excellent illustration of the porous nature of bauxite. A large 

 fragment was soaked for a day in water with gentle heating, and 

 gave a result of 2-487. Individual concretions from the same 

 specimen gave 2-53, while the mean of three determinations of the 

 fine powder gave 281. The last is taken as the true specific gravity 

 of the Katni specimen. 



1 Geol. Mag., Dec. IV, Vol. X (1903), p. 158. 



2 Eec. Geol. Surv. India, vol. xxxii (1849), p. 179. 

 2 Loc. cit. sup., p. 155. 



