Chap. XIX.] 
ORIGIN OF LATERITE. 
373 
the fresh underljang rock, with an intermediate stage of rock in a state 
of partial decomposition ; but that the laterite is a distinct formation 
resting on the underlying rock with no necessary connection between the 
two. This is the type of laterite that is often so aluminous as to be 
bauxite. It is also very free from siliceous matter, either free or com- 
bined. The best account of the structure and appearance of this tjrpe 
of high-level laterite is the account of the laterite of Bidar by Newbold l. 
Theories as to the Origin of High-level Laterite. 
The following is a list of some of the many theories that have been 
propounded to account for the formation of high-level laterite : — 
1. H. B. Medlicott and W. T. Blanford 2 in discussing this subject 
consider two possible methods of derivation : namely, ' that 
the high-level laterite is simply the result of the alteration 
in situ of various forms of rock, and especially of basalt, by 
the action of atmospheric changes and that it may be a 
sedimentary deposit. They conclude that in spite of the large 
amount of work that has been done on the subject, it is im- 
possible to say that either of these is the true theory, as there 
are objections to them both. 
2. F. R. Mallet 3 suggests the h}rpothesis that the high-level laterite 
may have been formed in lakes occupying shallow depressions 
formed on the surface of the Deccan Trap formation at the 
close of the period of its eruption ; during the decay of vege- 
table matter in contact with disintegrated ferruginous rocks, 
iron passes into solution as ferrous carbonate ; partly in 
streamlets on the way to the lakes, and partly after reaching 
them, the ferrous carbonate is subjected to oxidizing influences 
with the precipitation of hydrated ferric oxide ; whilst a 
further portion may be precipitated by the action of various 
organisms, especially algae, the pisolitic structure being due 
to deposition round a nucleus The mode of formation 
1 Jour. As. Soc. Bcng., XIII. pp. 989-994, (1844). 
2 ' Manual of the Geology of India', pp. 359-364, (1879). 
3 Rcc. G. S. I., XIV, pp. 145-148, (1881). 
4 Or to subsequent segregative action. With regard to the part that alga; may play 
in such a process it is interesting to note that according to G. IJertrand, Ecvuc Gcnrrnle 
de Chimie, VIII, p. 211, (1905), Jackson has described three sorts of Crenothrix. 
' C. Kiihniana (ou polyspora) qui separe I'oxyde de fer ; C. ochracea, qui separe Talumi- 
nium avec un peu de fer : et C. manganifira, qui separe le manganese'. To the originsl 
paper in Zeitsch, Untcrsuch. Nannuvjs-u. G. Mittel, VII, pp. 215-221, (1904), I liave not 
been able to refer. 
