Chap. XIII.] 
KODURITE SERIES ; DEPTH OF ORES. 
277 
a variable distance into the sea of underground water. The belt of cementation 
extends from the bottom of the belt of weathering to the bottom of the belt of 
fracture.' 
The distinction between these two belts is summed up by Van Hise ^ as 
follows : — 
'The belt of weathering is characterized by disintegration and decompositirn, 
carbonation, hydration and oxidation, by solution and decrease of volume. The 
belt of cementation is characterized by cementation and induration, by hydration, 
by deposition, and by increase of volume.' 
The chief constituent dissolved out of the belt of weathering and 
transferred to the belt of cementation is sihca formed during the decom- 
position of the silicates under the influence of water containing carbon 
dioxide. 
ij From this it will be seen that the chemical changes, namely, carbon- 
ation and oxidation, characterizing the manganese-silicate rocks of the 
Vizagapatam district must have taken place since the rocks of the 
kodurite series came into the belt of weathering. The depth to which 
„ , , , the zone of weathering extends at a particular 
Ground-water level. , t, t t ,^ , i r , 
locality depends on the level oi ground-water at 
that locaUty. In general the ground-water level follows roughly the 
inequahties of the earth's surface. According to Van Hise^ : — 
' In regions of moderate elevation and moderate irregularities of topography 
the level of groundwater is usually from 10 feet to 100 feet below the surface. It 
is especially likely to be near the surface in regions where there is a thick layer of 
drift or a thick layer of disintegrated rocks. In elevated and irregular regions, 
and especially those in which the precipitation is rather small, the level of ground- 
water may be from 100 to 300 feet below the surface. In high, desert regions, and 
especially limestone regions, the level of groundwater may be from a thousand 
to several thousand feet below die surface.' 
In the portion of the Vizagapatam district where the manganese- 
ore deposits are found, the topography is fairly flat, whilst the rainfall 
is moderate, namely 40 to 50 inches annually. Hence it is probable 
that the level of groimd- water is fairly near the surface, probably much 
less than 100 feet below plain level. Hence, if the deposits were formed 
whilst the ground had its present configuration, we should not expect 
them to continue to any great depth below the surface before giving 
way to moderately fresh examples of the kodurite series. In the 
Kodur mine, however, a depth of some 90 to 100 feet below the level 
of the adjacent plains has already been reached. Now, although the 
level of ground-water probably lies higher than the bottom of this 
pit — considering the large amounts of water that continually enter it and 
1 Ihid., p. 74. I 2 jbid., p. 61. • 
