Malcolm Maclaren — The Origin of certain Laterites. 539 



entirely from June to October, a period of four montlis. At Karwar, 

 for example, rain lias fallen on 72 successive clays, atid yet the total 

 number of rainy days in the same year has been no more than 120. 

 The average shade temperature of the same region is a trifle below 

 80° F., with a monthly range of from 75° to 84° F. With such 

 rainfall and temperature vegetation is naturally luxuriant, especially 

 in the lower-lying regions. During the cold weather, the dry field 

 season of India, little or no idea of the moisture precipitation of these 

 regions can be gained. Especially is this the case on the higher 

 lands. The Atli laterite plateau, north of Castle Rock, when seen in 

 January was a dry and arid, hopelessly waterless, area. Fortunately 

 the exigencies of shihar took the writer over the same area in late 

 July. The plateau, so far as it could be seen for the driving rain 

 and mist, was then, to outward appearance, a morass, though, through 

 its thin covering, one thankfully felt the hard laterite crust beneath. 

 Periods of dessication and saturation are thus very well marked. 



Regions of abundant rainfall in tropical climates are well wooded, 

 and humus being always abundant the proportion of carbon dioxide 

 in solution in the soil is correspondingly high. Boussingault and 

 Lewy^ show that, in a temperate climate, the air of a soil rich in 

 humus may contain more than one hundred times as much carbon 

 dioxide as atmospheric air. Considering the greater rapidity of 

 decay and chemical change in tropical regions, this factor is likely 

 to be greatly exceeded in India. The amount of silica dissolved in 

 underground waters may also be taken as a measure of the amount 

 of carbon dioxide in solution, and Struve' has shown that the 

 amount of silica thus dissolved is much greater in regions of 

 abundant vegetation than in those where it is absent or sparse. 



There is no need in this place to discuss at length the action on 

 silicates of waters containing alkaline carbonates and carbon dioxide 

 in solution. Bischoff', fifty years ago, suggested that the silicates 

 were decomposed by these agencies, and Kahlenberg and Lincoln ^ 

 have recently shown that the resulting silica separates in the form of 

 colloidal silicic acid. 



The normal weathering change of felspars in temperate regions is 

 undoubtedly to kaolin, as indeed is sufficiently indicated by the 

 analyses of the Rowley Regis dolerite by Dr. Warth ^ in connection 

 with this very subject. The general equation may be represented 

 thus : taking, say, albite as a typical felspar, 2 Na Al Sig Og + 

 10 Ho + C Oo = H.^ Alo Sio Og + Na. C O3 + 4 H^ Si 0^. Where 

 abundant carbonates are produced, as is apparently the general case 

 in humid India, the change is not to kaolin but rather to a sericitic 

 form : ' thus, 'd (Na K) Al Sig 0,^ + 10 H. + C 0. = (Na K) H. Al., 

 (Si O4) 3 + (Na K)3 C O3 + 6 H.^ Si 0^ ; and herein probably lies the 

 explanation of the general absence of kaolinite from the purely 



' Quoted by Memll : " Roclcs, etc.," p. 178. 



- Pon-g. Annul., vol. vii, pp. 341, 429. 



3 Joura. riiys. Chem., vol. ii (1898), pp. 77-90. 



i Geol. Mag., Dec. V, Vol. II (1905), p. 21. 



^ Lindgren : Trans. Anier. Inst. M.E., vol. xxx, p. G14. 



