62 T. H. Holland — Constitution of Laterite. 



is due probably to mechanical sorting by running water, the light, 

 scaly gibbsite being carried away, whilst the heavier ferric hydrate 

 remains with the grains of sand. Thus, taught by the natives who 

 sometimes use it as an iron-ore, we have been blinded to the true 

 nature of laterite by keeping an eye on its possible use as a source 

 of iron. That the Seychelle specimens are of the ' high-level ' type 

 seems probable from the fact that the ratio between alumina and 

 ferric oxide in each case corresponds approximately with that of the 

 rocks on which they lie, granite in one case and diorite in another. 

 They retain, too, just as much silica as one would naturally expect 

 to occur as free quartz in the original rocks. 



With laterite one ought also to consider the material known 

 generally as hunTcar, without much respect to the original meaning 

 of the Hindustani word. Laterite and Tcunkar appear to be formed 

 by precisely the same process ; but they differ from one another in 

 composition, and the reason for this is seen in South- West India. 

 The Western Ghats of Malabar divide areas which differ greatly in 

 rainfall. The south-west monsoon, breaking against the hills of 

 Travancore and Malabar, precipitates between 100 and 200 inches 

 of rain on the western slopes during the wet season ; whilst on the 

 eastern side the plains of Coimbatore receive no more than about 

 20 inches of rain during the year, and the field-worker in Coimbatore 

 sees with satisfaction the great bank of cloud spending its fury on 

 the Ghat ranges, whilst a cool breeze blows through the Palghat 

 gap, and spreads itself over the eastern plains with seldom more 

 than a sprinkling of rain. As we pass from the west to the east 

 side of the western ridge, there is a complete change in the 

 weathering products. On the western side, where running water 

 is abundant, there is the crust of typical ferruginous laterite. On 

 the east side, where the climate is quite as moist, but actual pre- 

 cipitation of water very limited, the ferruginous character is masked 

 by the great predominance of lime-carbonate, and the material, 

 from its resemblance in composition to the nodular bodies in the 

 Gangetic alluvium, is known to the Public Works officer as hunJcar, 

 being similarly used as a source of hydraulic cement. In Tcunkar 

 we have the total products of decomposition ; exposure of this to 

 running water would remove the lime, and leave a residue indis- 

 tinguishable from laterite. Both substances thus originate in the 

 same way by the decomposition of rocks in a tropical, moist climate. 



Laterite is characteristic of the tropical belt, and so, too, is 

 JciitiTcar; but this possibly is merely a case of intensity : both may 

 be formed to a less noticeable degree in temperate regions. Still, 

 in their typical forms these peculiar substances are eminently 

 characteristic of the tropics — one of a wet and the other of 

 a humid climate. 



That the formation of laterite is a mere question of average 

 temperature seems unlikely, for lateritization is very prominent at 

 elevations of 6,000-7,000 feet in the Nilgiri and Palni Hills in South 

 India, where the temperature varies very little above or below 

 60° F. The concretionary structure so characteristic of the laterite 



