Chap. XIX.] 
ORIGIN OF LATERITE. 
379 
seems to be some such passage. Moreover, several other cases of 
apparent passage of underlying rock into laterite have been noticed by 
previous writers, an especially clear example being the one recently 
figured by Mr. Maclaren (see page 38^). 
It is precisely with regard to the nature and reality of such passages 
that opinions differ. It is generally accepted that the materials com- 
posing the type of laterite now being discussed have been derived in 
some way from pre-existing rocks, usually basic lavas of the Deccan Trap 
formation. It is also generally recognized that the formation of this 
type of laterite involves the disappearance, probably in solution, of 
the silica, lime, magnesia, and alkalies, of the original rock, with the 
concentration of the oxides of aluminium, iron, titanium, and sometimes 
Origin of the Yeruli manganese, to form laterite ; this result of general 
laterite. experience agreeing with the particular case of 
Yeruli. as illustrated by the analyses given above. But, as can be 
seen from pages 374-5, there are many theories as to the method by 
which this result may have been produced. Each of these explanations 
may be true as applied to some particular deposit of laterite ; but in 
many cases a combination of them will probably be nearer the truth. 
In the particular case of Yeruli it seems necessary, as already explain- 
ed, to adopt Mallet's explanation with regard to the uppermost layer of 
laterite, namely that containing pebbles of bauxite. But, because of 
the difficult of accoimting for the alteration of the underlying rock, with- 
out hypothecating the downward percolation of mineralized solutions, of 
which there is no sign, this explanation cannot be extended to the whole 
mass of laterite. For exactly the same reason Wetherell's theory is 
put out of court. This leaves the theories of Holland and Maclaren. It 
is very difficult to say which, if either of these theories applies to the case 
in question. Seeing, however, that the altered lavender-grey rock inter- 
vening between the laterite and the underlying trap is shown by analysis 
to be particularly aluminous and thus to correspond in chemical character 
with the prevailing nature of the overlying laterite, I am inclined to think 
that the rock was decomposed in situ with a gradual passage into the 
aluminous laterite. Had the laterite been formed in accordance with 
Maclaren's theory, there is no reason why the aluminous laterite should 
have been deposited just where the underlying rock happens to be 
particularly aluminous. For the rocks of this area are at least as ferru- 
ginous as aluminous, and consequently the circulating solutions that 
formed the laterite cap by replacement of the decomposed rock would 
probably have deposited a more ferruginous type of laterite. Whether, 
