380 
MANGANESE DEPOSITS OP INDIA : GEOLOGY. [ PaRT ll : 
on the other hand, it is necessary to attribute this formation of laterite in 
situ to an organic agency, as suggested by Holland, instead of to the action 
of ordinary chemical processes due to weathering, as in the usual theory 
of formation by alteration in situ (as noticed by Medlicott and 
Blanford), I do not feel in a position to judge. Apart from this point, 
we arrive at the conclusion that the lower portion of the laterite of Yeruli 
was formed by the decomposition of basic lavas in situ, and the upper by 
deposition in lakes or bogs, after a period of denudation. 
I have discussed this case in such detail, partly to indicate the ex- 
Necessity of considering difficulty of the question of the origin of 
each occurrence of laterite laterite, and partly to indicate that, in all pro- 
separateiy. bability, no oue explanation will apply to all 
varieties of high-level laterite, and that it is necessary to consider each 
case separately. Perhaps, after a considerable number of cases have 
been so discussed, we shall be in a position to form a truer idea as to 
which of the various theories here considered accounts for the greater 
proportion of the occurrences of high-level laterite. 
Lateritic Manganese-ores. 
But whatever the mode of formation of this high-level laterite may 
Result of formation of be, the result, already stated on page 370, is 
laterite. certain, namely the formation of masses of rock 
composed essentially of oxides of iron, aluminium, and titanium, usually 
associated with a large amount of combined water ; the silica, lime, 
magnesia, and alkalies, of the rock from which the former constituents 
The destination of man- '^'^re derived having been eliminated. Up to 
ganese when laterite is the present I have omitted to refer to what 
formed. becomes, during the formation of laterite, of the 
small quantities of manganese that are contained in almost all the rocks 
of the earth's crust. When, in the course of the decomposition of the 
rocks in the area where lateritization is going on, the oxides of iron and 
aluminium pass into solution i, it is probable that soluble salts of man- 
ganese are also formed and dissolved ; these salts being in all probability 
either the bicarbonate, or combinations with various organic acids such 
as are often found in soil where vegetable matter comes into play. As 
I have already noted in a paper^ treating of the association of gibbsite 
1 It is to be noticed that, even in the case of laterite formed by decomposition of 
rock in situ, the oxides of iron and aluminium must have passed into solution at least for 
a short while, as is evidenced by the fact that laterite seems always to have been 
reconsolidatcd tlirougiiout and chemically rearranged. 
2 Rtc. 0. S. I., XXXIV, p. 168, (1906). 
