278 
MANGANESE DEPOSITS OP INDIA : GEOLOGY. 
[Part II : 
necessitate constant pumping — , yet good ore is found to the very 
deepest parts of the quarry. Hence it would seem that the present 
ground-water level is higher than it can have been at the time of 
formation of the manganese-ores, so that we cannot rely on the 
present level of ground-water for information as to the depth to which 
the manganese- ore deposits may be expected to continue. 
This means that the main mass of the ores was formed in the past, 
Time of formation between the time when the processes of elevation 
of ores. and denudation brought the rocks of the kodurite 
series to the surface and the present. The last serious folding of 
the Archaean rocks of this area was probably contemporaneous with the 
folding of the Dharwar rocks in other parts of India, which took place 
somewhere between the deposition of the Dharwars and the rocks of 
the Purana systems. It is probable that during this folding some of the 
rocks of the kodurite series were exposed at the surface. Their altera- 
tion may therefore have commenced in Archaean times and continued 
ever since, except for interruptions when the outcrops of the Archaean 
rocks were covered up by younger formations, such as perhaps the 
Purana formations and the Deccan Trap. No doubt denudation has 
been continuously removing the surface portions of the altered rocks, 
thus bringing deeper and deeper zones of the rocks nearer the surface 
and consequently within the influence of the carbonating and oxidizing 
waters. 
Now the depth to which this alteration extended in past times depends 
Depth to which the 0^ the maximum depth of the ground-water during 
ores extend. the period in which the masses of kodurite rocks were 
exposed at the surface. In the absence of any information as to the 
climatic and topographic conditions of this region, between Archaean 
and historic times, it is not possible to say what was this maximum 
depth of ground-water in past times. Unless, however, this region 
was ever an arid region of very irregular and elevated topography, it is 
not likely that the ground-water level ever sank to the extreme depth of 
.1 000 and more feet characteristic of such regions. Let us assume that 
it was never at a depth of more than about 500 feet. Now, according 
to Van Hise, the zone of weathering may extend for some distance 
below the ground-water level. To allow for this let us put the 
maximum former extension in depth as 700 feet. Allowing for the denu- 
dation of an unknown amount of the surface portion of the kodurite rocks 
since the time when the ground-water level reached this maximum 
