Chap. XlY.] 
DHARWAR SERIES t DEPOSITION. 
^86 
been quite small. These basin-like areas were areas of deposition, 
both mechanical and chemical. Sometimes the incoming waters were 
comparatively clear and free from mechanically- carried material, such 
as sand or clay, but, instead, contained in solution salts of manganese. 
On reaching the masses of more or less stationary water the velocity 
of the incoming waters was checked, and, in the more or less still 
condition to which they were brought, the solutions were either sub- 
jected to atmospheric oxidation v/ith the consequent formation and 
precipitation of oxide of manganese, or they were subjected to the 
action of reducing agents with the precipitation of the manganese in 
form of carbonate, or, much more improbably, of sulphide, according 
to the nature of the reducing agent. It is considered that the former 
alternative is in all probability the correct one, although the latter mode 
of deposition, namely, as carbonate, and possibly in very rare instances 
as sulphide, may have been locally operative, where the conditions of 
deposition were reducing instead of oxidizing, as must usually have 
been the case. As long as the incoming waters were free from silts the 
deposits accumulating on the bottoms of the basins must have been 
entirely chemical and composed of more or less pure oxides of mangan- 
ese. But when the waters contained a moderate amount of silt, as well 
as the manganese salts, then the manganese oxides deposited must have 
been rendered siliceous by mechanical admixture with sand or clay. 
At other times, there must have been an alternate deposition of mangan- 
ese oxides, either pure or impure, with layers of sand or clay partly or 
entirely free from manganese oxides ; the layers of silt being deposited 
at times of flood, when the incoming waters had so diluted the man- 
ganese-bearing solutions with waters charged with silt that the sediments 
deposited consisted almost entirely of sand or clay. After the finish 
of the period during which manganiferous sediments were deposited, 
there came a time when the supply of manganese-bearing solutions 
ceased and the whole succession of manganese-bearing sediments was 
buried beneath a great thickness of sands, clays, and other sediments. 
The source of the The manganiferous solutions themselves must have 
/i-anganese. been supplied from an area in which the rocks 
then subject to meteoric influences contained compounds of 
manganese liable to be decomposed under the action of such influences. 
For such a source we can probably look to some plutonic rock 
containing manganese-bearing silicates amongst its constituent 
minerals. The ordinary ferro-magnesian silicates usually contain small 
quantities of manganese, and this would no doubt be sufiicient under 
