382 
MANGANESE DEPOSITS OF liSDIA ; GEOLOGY. [PaRT II : 
1,300 to 1,400 feet. The remainder occur in a rock that some geolo- 
gists would probably, designate ' laterite ' ; but others would pro- 
bably object to the application of the term. The rock to which I 
refer has a lateritic aspect and usually consists of a cavernous mixture 
of various oxides ot iron, chiefly hard limonite, yellow ochre, and soft 
hematite. When no other constituents are present, the rock often 
resembles typical laterite in its structures and mineral composition so 
closely that when detached from its rock masses it could not be 
distinguished from pieces of ordinary ferruginous laterite (of non- 
detrital origin). Fairly often, however, it contains ores of manganese, 
either wad, psilomelane, or pyrolusite. The iron-ores and manganese- 
ores are mixed with one another in very irregular manner. Veins 
and patches of the manganese-ores sometimes occur in the iron-ores, 
whilst in other places veins and patches of iron-ores occur in a mass 
consisting chiefly of manganese -ores. Owing to the cavernous charac- 
ter of the rock the limonite and psilomelane have often been able to 
develop marked concretionary structures, such as botryoidal or stalac- 
titic, whilst the pyrolusite is often in small crystalline aggregates. 
This rock does not always consist entirely of iron and manganese- 
ores. It often contains patches of quartz, quartzite, slate, or phyllite. 
Examination with the microscope shows that these rock patches are 
residual pieces of rock set in a matrix of ore, and that the latter has 
evidently been formed by their replacement. Examination of the 
masses of rock in the field, especially as revealed in the workings for 
manganese, confirms this deduction, and shows that there is a down- 
ward passage from the lateritic mass of iron- and manganese-ore at the 
surface through rock containing more and more quartzite, slate, or 
other rock, and less and less ore, to a rock that is free from all signs 
of ore The junction between the overlying lateritic mass of rock and 
the rock on which it rests is consequently an extremely irregular one. 
There is, in my opinion, no doubt that these lateritic masses have 
been formed by the mctasomatic replacement of the quartzite, slate 
or phyllite, accompanied by segregative changes. And as these rocks do 
not usually contain uK.re than a very small proportion of, and often no, 
manganese, it is evident that the manganese must have been largely 
brought in from outside by percolating waters, with the resultant 
repla ement from the surface downwards of the particular rock that 
happened to be at the surface where the replacement took place. 
These lateritic replacement deposits nearly always occur as cappings 
to small hills, the actual rugged surface of the rock (often spotted 
