Malcolm Maclaren — The Origin of certain Laterites. 545 



The chemical analysis shows that at least 15 per cent, of the mass 

 has, in the form of ilraenite and possible rutile, a specific gravity of 

 more than 4-3. The remaining 85 per cent., therefore, has a specific 

 gravity of not less than 2'65. But an intimate mixture of 71 per 

 cent, diaspore and 29 per cent, gibbsite should have a specific 

 gravity of 303. Rough though the calculation is, it is nevertheless 

 sufficient to show that the two lines of deduction are hopelessly 

 conflicting, and it must either be assumed that diaspore is not present 

 or that the monohydrate, if present, is in a form physically difierent 

 from diaspore. Additional evidence against diaspore lies in its 

 absence from all thin sections examined. 



Fig. 2. — Thin section of portion of a Bauxite concretion, Katni, Jabalpur, showing 

 gibbsite crystals (colourless) in bauxitic (dark) groundmass. 



A typical thin section of a concretion from the Katni rock throws 

 considerable light on the structure and internal characters of bauxite 

 (Fig. 2). The mass of the rock is made up of white opaque 

 bauxite with a well-defined banded structure near the exterior of the 

 concretion. Scattered through the interior and avoiding the outer 

 band, are numerous irregular areas of gibbsite in rosette-like 

 aggregates. Dispersed everywhere irregularly through the interior 

 of the section, but disposed in more or less linear arrangement in 

 the outer band, are numerous minute black and brown specks, too 

 small, indeed, to be determined with ease or accuracy. The black 

 grains are, however, almost certainly ilmenite, while the browu 

 granules may be rutile, but, from comparison with other sections, 

 are thought to be more probably the clove-brown micaceous ilmenite. 



A consideration of the disposition of the gibbsite indicates clearly 



BECADE v. — VOL. III. — NO. XII. 35 



