PART 2 .] 
Medlicott, Metamorphic rocks of Bengal. 
45 
depression at the time of the first great granitic invasion; and that to this we may owe 
their ultimate preservation as hill ranges. The feature is well exhibited in Mahabur: the 
axis of the range would strike up a reach of the Sukri, and on each side of the river there 
is an elongated oval of granitoid rock, as of denuded domes. The characteristic arrangement 
of the doubtful gneissic rocks is also better shown here than anywhere I could mention: 
they appear as regular concentric coatings to the granitoid masses. The area on the south 
is less elevated, and the diverging dips of the covering rocks range from 5° to 20°; on the 
northern area they are much steeper as if exposed lower down on the sides of the mass they 
envelope, but the regularity of the encircling ring is unbroken, and of the same description 
of rock as on the south, but in thicker masses. Upon the usual evidence of regularly alter¬ 
nating stratification of highly contrasting materials, it is not, I presume, to he questioned 
that these are true detrital accumulations remineralized; and it might, I suppose, be 
maintained on the strength of some misunderstood process of hypogene intrusive action that 
they may have underlaid the Rajgir formation; or it might even be said that the strati- 
graphical features of the Mahabur region suggest such a relation. But from the evidence 
before us, I confess to a preference for the contrary supposition : it would require that after 
the great disturbance and metamorphism of the Rajgir series the whole area was 
denuded to a much greater extent than now, and that upon the surface thus exposed 
these accumulations took place, probably of some arkose-like materials, very susceptible to 
mineral reorganization. Such must have been the composition of the bottom-infra-B i j aw u r s .* 
In this Behar area, however, there is ample evidence of a later granitic invasion: in the 
southern tributaries of the Sukri there are fine sections of great granite dykes traversing all 
the rocks transversely. This granite is very different from that already noticed; it is highly 
crystalline; in the centre of the dyke the felspar and quartz form a coarse graphic granite, 
with associated schorl and beautifully plumose mica. The view I have proposed would imply 
a prodigious relative antiquity for the Rajgir formation. 
The views that have now been presented in connection with the submetamorphic 
series have manifestly very direct bearing upon the rocks of the great gneissic area. Suppos¬ 
ing the conjecture regarding the extensive representation of the peculiar infra-Bijawurs to 
be correct, there would be four principal geological divisions to be discriminated and 
mapped, exclusive of all later granites, &c. 1st. There would be the gneissoid granite, 
which would seem to be largely present, to he distinguished from true metamorphic gneiss. 
I should despair of settling this point without the extensive application of microscopical 
analysis of the rocks ; indeed it remains to be seen whether even this test would furnish 
a criterion, whether the crystals of such a rock would not assimilate more to metamorphic than 
to fully igneous products, 'lad. It is more than probable that associated with that granite we 
■should find a most ancient gneissic formation long anterior to the metamorphism of the 
Rajgirs, and possibly equivalent to the gneiss of Bundelkund. 3 rd. We should probably 
find remnants of the Raj girs in their gneissose form. On this point there is some inform¬ 
ation at hand : far within the great crystalline area, near the Grand Trunk Road north of 
Burhi, there is an inlier of typical Mahabur (Rajgir) schists. If they always remain so 
characteristic there will be no difficulty in recognising them. Even here they are attended 
by the encircling ring of variable quartzites, having high converging dips towards the 
schists, which occupy the lowest ground in the neighbourhood on the banks of the Barrakar. 
The quartzites form a narrow ridge round them, and would belong to our next division. 
4£/t. There would be the hypothetical infra-Bijawurs. The establishment of this series 
would probably relieve our field work of some perpetually outcropping difficulties, especially 
in the shape of isolated, discontinuous runs of quartzites and breccias. But apart from these 
more characteristic beds, I could not now assign a lithological criterion for this series gen¬ 
erally : as has been seen they even simulate granitic masses. Great irregularity and discon¬ 
tinuity is one of their features; although frequently presenting excessive contortion, as if when 
caught between two resisting masses, they are generally comparatively little disturbed; and 
what disturbance they exhibit seems to be largely determined in direction by local circum¬ 
stances, resulting in great irregularities of dip. In Bundelkund, where they were first detected, 
these beds seem to have but little extension ; but in Bengal they seem to occupy large areas : 
I have observed rocks of this description in far distant localities of the great gneissic area. 
Janumy 1869. 
* As a more recent parallel for such kind of deposits, I would refer to the felspathie beds of the lower Vindhyans 
as exposed in western lfehar, to south-west of Kutumbeh. 
