PART 1.] 
Mallet: Geological Nofee. 
41 
Occasionally the granite occupies considerable areas, as south of Pi hr a, where in a large 
mass of irregular hills no other rock is seen, and again south of G a w a n; for a mile below 
Sankh the Salcri river flows continuously through it. Most commonly, however, it is 
found penetrating the older rocks in dykes and veins, varying from 50 yards or more in thick¬ 
ness, down to less than an inch. These are more usually roughly parallel to the foliation of 
the rocks they penetrate, but by no moans universally so. Many cut through quite obliquely 
and irregularly, and ramify in various directions. They are mostly vertical or hade at high 
angles (agreeing in this respect with the foliation), but instances occur where they arc nearly 
horizontal. Such horizonal dykes of greater thickness than usual may perhaps, in some cases, 
give rise to the larger granitic areas. 
The surrounding rocks seldom present much appearance of alteration in the vicinity of 
the granite; generally the junction is quite sharp, and the beds in immediate contact not 
different from what they are at a distance. It is to be remembered, however, that these beds 
had perhaps already undergone metamorphism before the introduction of the granite. Some¬ 
times there is a rapid passage of a few inches from one rock to the other, as if the strata in 
immediate proximity had been fused or greatly softened. South of Pi hr a, the mica-schists 
south of the large spread of granite there, pass into gneiss near the junction; but as there 
is always a gradation from the mica-schist into the true metamorphics in this portion of the 
country, it does not follow that the above passage is in any way connected with the granite. 
After an examination of the granite in the innumerable dykes and veins, presenting 
every appearance of having forced its way in uneven and ramifying courses through the 
circumjacent strata, one can scarcely avoid feeling satisfied as to its truly intrusive origin, 
and the fact of its maintaining a constant mineral character amongst the different rocks 
through which the dykes occur, whether these be gneiss, mica-, hornblende-, or quartz- 
schist or even limestone, some of which are rocks from which the granite could not possibly 
have been produced by any mere chemical re-arrangement, leaves no escape from this view. It 
is, however, on the other hand, not easy to explain the occurrence of thin strings of granite 
of an inch or half an inch thick, running exactly parallel to the foliation of the including mica 
schist for many yards, and at a distance from any visible dyke, and of lenticular pockets 
(also parallel to the foliation) completely isolated to all appearanco in the surrounding rock. 
Such cases are so very common, that I scarcely think it is always a sufficient explanation to 
say that they are offshoots of some dyke hidden beneath the surface, or were once connected 
with a dyke above, which has since been removed by denudation. I have already alluded to 
the passage of the granite into vein quartz. These arc points requiring elucidation, but that 
the granite is really intrusive is beyond question. 
The dykes are far more plentiful in the submotamorphic than in the metamorphic series, 
and their distribution in the former is very unequal. In some areas they occur, large and 
small, by hundreds; in others they are entirely wanting. 
Mica Mines .—It is in this granite that the well-known mica mines of Bihar* and 
the neighbouring districts are situated. I have previously said that the coarsest pegmatite 
is frequently found in dykes of moderate thickness, in which, therefore, plates of mica of the 
largest size occur, and it is such dykes that the miners generally select for their operations. 
They pay from one to two rupees each per annum, according to the ricknoss of the yield, 
to the owner of the land for the privilege of mining. The usual mode of working is simply to 
excavate a trench along the course of the dyke, which in the G a w a n neighbourhood is 
seldom carried deeper than 20 or 25 feet. Sometimes where there is a considerable thickness 
* A paper by Captain Sherwill on the mode of working the mica mines in the B i h d r district may be found in 
the Journal Asiatic Society, Bengal, vol, XX, p. 295. 
