64 T. H. Holland — Constitution of Laterite. 



bacillus at work. There may be many forms of life taking advantage 

 of the soft, moist, lateritio medium, but it will not be an easy 

 matter to convict, amongst these, such as may take an active part 

 in breaking up the aluminous silicates. 



It is hard to believe that the few degrees by which a tropical 

 exceeds a temperate climate is sufficient to so strikingly increase 

 the chemical activity of the weak organic acids percolating through 

 the soil. But that such a small difference of temperature affects 

 low forms of life is painfully evident to those who have to maintain 

 the daily fight of life in the tropics. The enormous depths to which 

 the complete destruction of rocks extends in moist, tropical, climates 

 has frequently been described by travellers ; but no description 

 reproduces the actual impression caused by the deep cuttings in 

 places like the hill-roads of South India, where, on a newly-cut 

 face the minute structures of the gneisses are perfectly preserved in 

 material as soft as putty. And yet, as I have pointed out elsewhere,^ 

 there is a sharp and sudden passage from the soft decomposition- 

 product to a rock so fresh that not a sign of alteration can b© 

 detected in it under the microscope. This is true, too, of boulders 

 isolated from the main mass of rook and embedded in the decom- 

 position-product : within the distance of an inch one passes from 

 the soft material, which we have unwittingly called clay, to ideally 

 fresh, water-clear felspars. 



If this alteration were due to the mere infiltration of water 

 through the rocks one would expect to find signs of incipient 

 alteration to greater depths in the solid rock. But if it is due, as 

 I suspect, to a growth at the surface, the sharp line of junction is no 

 more than we should expect. Darwin, against whose observation 

 nothing was secure, and who never observed without deduction, 

 thought the thick decomposition mantle on the Brazilian coast was 

 due to hydration under submarine conditions.- But that, we are 

 certain, is not the case with the South Indian hill-ranges ; the action 

 is purely subaerial, and the sea has never been near the rocks. 



But the existence of low forms of life capable of decomposing 

 aluminous silicates would be no surprising discovery after what we 

 know of the action of the so-called nitrifying bacteria in converting 

 inorganic salts into proteids for the use of higher plants ; of those 

 which, contrariwise, break up nitrogenous organic matter to form 

 nitrates ; of the peculiar anaerobic forms which break up sulphates 

 and separate the sulphur, and of the aerobic forms which convert 

 sulphides into sulphates. 



Amongst the forms almost certain to occur in newly-forming 

 laterite will be relatives of the remarkable genera Crenothrix, 

 Gladothrix, etc., which are active agents in the oxidation of ferrous 

 salts, depositing the insoluble form of ferric oxide in their cell- walls 

 as a part, apparently, of the physiological activity of the organism. 

 There are probably bacteria akin to these which assist in the 



1 Holland, "The Charnockite Series " : Mem. Geol. Surv. Ind., 1900, vol. xx-\dii, 

 p. 197. 



2 Voyage of the " Beagle," 2nd ed., 1876, p. 427. 



