Malcolm Maclaren — The Origin of certain Laterites. 541 



underlying or adjacent rock masses, in solution, and probably in 

 ionized solution, all tlie materials essential to the formation of 

 laterite. There is now to be considered the machinery necessary 

 to bring it to its resting-place at the surface. Tlie conditions of 

 successive dessication and saturation in regions of lateritic growth' 

 have already been indicated. During the former period the motion 

 of ground waters is always from the region of greater to the region 

 of lesser humidity, or, in this case, upward to the surface. Upward 

 transmission is effected entirely by molecular attraction, resulting, 

 where conditions are favourable in capillary motion, that directly 

 due to dessication being intensified by the needs of forest rootlets. 

 The potency of capillary transmission may be gathered from the 

 experiments of King,^ who showed that molecular attraction, acting 

 in capillary openings in a porous rock, was capable of raising from 

 a depth of four feet an amount of water equivalent to an annual, 

 rainfall of 63-85 inches. The larger tubes anastomosing in laterite 

 have often been described, and microscopic examination of both 

 ferruginous and aluminous laterites discloses numerous tubular 

 openings both capillary (-0002- "508 mm.) and super - capillary 

 (exceeding '508 mm.). 



With the approach of ground waters to the surface and their 

 consequent super-saturation in the dessicated region, separation of 

 the contained solids takes place, at first, it would seem, on the tube 

 walls and around pisolites. The minute tubes in the matrix of most 

 laterites render it probable that some moisture passes entirely 

 thi'ough the laterite crust and that a slight deposition takes place on. 

 the laterite surface exposed directly to the atmosphere, but by far the 

 greatest amount of deposition must take place on the under side of 

 the hard surface crust. During the dry season the whole of the 

 contained load of the upward moving waters is thus deposited. 

 There is at that time in the dessicated rock and soil but little 

 metasomatic replacement, though segregation may readily take place 

 as the solutions become more and more concentrated. During the 

 period of saturation, however, all soluble constituents deposited 

 during the previous dry season are dissolved and carried away. 

 The insoluble constituents are thus left to obey in a moisture- 

 saturated soil their natural laws of growth. They are further 

 segregated, and deposited in most cases as fine bands around already 

 formed concretions. Segregative action is well marked in the 

 Talevadi sections, the lower zones, nondescript in colour and com- 

 position, passing upwards into well-defined ferruginous laterite, 

 manganese ore (mainly psilonielane), pure bauxite, and gibbsite. 



In the ordinary course of formation of laterite a surface crust is 

 first formed which increases in purity, hardness, and depth with 

 every successive dry season. The depth and hardness of the hard 

 unaltering layer may, therefore, in areas in which laterite is still 

 being formed, be some measure of the age of the deposit. All 

 stages are met with. At Talevadi the hard crust is two to three 

 feet thick, in the Central Provinces it is often 20-30 feet in. 



1 U.S. Geo!. Surv., 19th Anu. Kep. (1899), pt. ii, p. 85. 



