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62 COAL.
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The
thickness attains its maximum in the Jeriah coal field, where it is
estimated to be 3,800 feet. In the Ranigunj field it is 2,000 feet; in
most of the other fields it is much less.
Ironstone Shale Group.—This group, consisting of bands of ironstones, running through grey and black
(carbonaceous) shales, overlies the Barakar group with general
conformity. It is only found in the Damuda Valley fields, wholly
disappearing further west.
In the Bokaro field it attains its maximum thickness of 1,500 feet.
Ranigunj (Kamthi) Group. —
The Ranigunj group consists of sandstones—which are fine-grained and
often calcareous—carbonaceous shales, and coal. The coal is generally
of better quality and more uniform in composition and in the thickness
of seams than is that of the Barakar group. In the easternmost field of
the Damuda Valley series—namely, the Ranigunj, which has given
the name to the group—the principal coal seams which are worked belong
to this group. In the more western fields it steadily thins out, the
coal becoming of less and less importance.
In
the central fields of the Peninsula it is very much changed in
lithological characters, and is so greatly increased in thickness,
amounting to from 5,000 to 6,000 feet, that the true identity with it
of these latter deposits which constitute the so-called Kamthi group is
established only by general geological relations aided by fossil
evidence.
The
rocks of the Kamthi group are largely made up of coarse sandstones and
conglomerates, in which there is a prevailing reddish colour due to the
amount of iron always present. Coal rarely occurs as a member of this
group ; its importance is insignificant.
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