t 22 ] 



8th. Laterite. The origin of laterite and its position in 

 the geological system are subjects of some di'spute. Laterite 

 is porous argillaceous rock much impregnated with iron 

 peroxide, some containing as much as 25 to 35% of iron. 

 The iron exists chiefly as limonite or hydrated peroxide. 

 The surface of laterite after exposure is covered with a brown 

 or blackish brown crust of limonite, but the rock when freshly 

 broken is mottled with tints of brown, red and yellow and a 

 considerable proportion consists of white clay which contains 

 no iron. The exposed surface is pitted with hollows and 

 irregularities caused by washing away of softer portions. 

 The rock has a scoreaceous and volcanic appearance, especial- 

 ly as it is associated usually with basalt and other igneous 

 rocks But it is now usually believed to be of detrital 

 origin produced from other rocks, igneous and sedimen- 

 tary. The high-level laterite of Central and Western 

 India does not appear to be detrital in origin as the 

 iron is not sandy. The low-level laterite of Bengal is 

 mixed up with sand, quartz, pebbles, ferruginous sandy 

 clay and gravel. The high-level laterite always caps the 

 highest lava flow, which makes the subject of its origin 

 so difficult to understand. Probably the laterite has been 

 formed from the disintegrated tuffs and scoriae, rearranged 

 by the action of water. As laterite resists the disintegrat- 

 ing action of the atmosphere longer than any form of Basaltic 

 rock, the high-level laterite still caps the basalt. The low- 

 level laterite is probably the detritus of the high-level laterite. 

 The action of rain and streams having carried away the 

 lighter sand and clay, the heavy iron sand is left as laterite 

 and to this may be due the concentration of the ferruginous 

 element. The age of the low-level laterite is certainly post- 

 tertiary, though that of the high-level laterite is uncertain, 

 the origin of this laterite being uncertain. 



yth. Alluvial deposits and Blown sands. Blown sand 

 forms the soil of places close to the sea, and its deposit 



