Ixxxvi 
MANGANESE DEPOSITS OF INDIA. 
sometimes a large percentage of manganese oxide, although laterite 
is often quite free from this constituent. When iron oxides 
predominate the rock may be of value as an ore of iron ; when it 
consists chiefly of oxide of aluminium it is known as bauxite and 
is of use as an ore of aluminium; whilst when manganese oxides 
predominate, usually in the form of psilomelane, pyrolusite, or wad, 
the rock is an ore of manganese. 
Laterites can be divided into two main divisions according to their 
situation, namely low-level laterite and high-level laterite. The only 
authenticated occurrence of manganese in low-level laterite is in Goa. 
The high-level laterite is found in its best development on the Deccan 
plateau, and in the Central Provinces and Central India. It is usually 
confined between the elevations of 2,000 and 7,000 feet. Numerous 
theories have been advanced to account for the origin of high-level 
laterite ; of these the following are the most important : — 
1. H. B. Medlicott and W. T. Blanford consider two possible 
methods of formation : namely that the rock has been 
formed as the result of the alteration in situ of various 
rocks, especially of basalt, and that it may be a sedimen- 
tary rock. They give objections to both hypotheses. 
2. F. K. Mallet supposes that the laterite may have been formed 
in lakes occupying shallow depressions on the surface of 
the Deccan Trap, at the close of its period of eruption, 
by being precipitated as oxides of iron from waters drain- 
ing into the lakes, partly by the action of oxidizing 
influences, and partly as the result of the vital activity of 
organisms, such as algae, living in the lakes. 
3. T. H. Holland suggests that the rock is formed in situ, as the 
result of the chemical alteration of rocks, the energy 
necessary for the breaking up of the siUcates being derived 
from the vital activity of organisms. 
4. E. W. Wetherell supposes that the laterite of the Bangalore 
and Kolar districts in Mysore is of detrita! origin and was 
formetl by the washing of the decomposed surface detritus 
of the svrrounding elevated ground into a lake, where it 
became mixed with non-detrital material to a small extent. 
