t)E(X)AN THAP. 
158 
ing into the water, and judf^inji from such outcrops of f,'ranitic 
rocks as protruded above water there was no reason why the 
dyke should not cross the river and connect uj) with the dyke- 
ridge seen clearly on the ojjposite bank. 
As in the case of the dykes, so also in that of tlie sill, the 
doleritcs are composed of the minerals characteristic of the Deccan 
Trap surface lavas, namely augite, plagioclase, and magnetite, 
frequently with interstitial patclies of partly devitrified and altered 
glass. Olivine is present more frequently than not, often in abun- 
dance ; but, curiously enough, the coarsely crystalline centre of 
the sill is usually devoid of olivine, as if sufficient time had elapsed 
after the intrusion of the sill and before its sohdification, except 
at the actual margin, to permit of a degree of magmatic 
differentiation, in which the olivine crystals or the olivine-forming con- 
stituents had migrated from the centre of the sill towards its more 
peripheral portions. The central dolerite is so coarsely crystalline that 
it would be termed a gabbro were it not for the ophitic relation- 
ships of the augite and plagioclase and also the existence of the 
devitrified interstitial patches already mentioned. These dolerites 
are clearly of considerable petrographical and petrological interest, 
rendering it desirable to defer their detailed treatment to a future 
occasion : the same course seems necessary with reference to a 
series of volcanic rocks of uncertain age provisionally designated 
the Chirmiri volcanic series and to which a brief reference must 
now be made. 
About a quarter of a mile north-east of the most easterly 
hamlet of Chirmiri village on the road to 
Chirmiri volcanic Kurasia, there is an outcrop of a curious 
series. . ' . i . 
volcanic rock traceable at intervals in a north- 
north-westerly and southerly direction for about three quarters of a 
mile. It is a vesicular rock apparently approximating to ar 
andesite in composition ; and from its vesicular nature one 
would deduce it to be a surface lava. Its linear distribution, 
however, suggests a dyke, and this supposition agrees with the 
fact that associated with it are included fragments of Barakar 
sandstones and shales. There are also admixtures of the two 
rocks, in which grains of detrital quartz are scattered through 
an andesitic matrix, whilst in addition there are banded hornstone- 
like masses of rock suggesting shales indurated by heat. All the 
facts mentioned above, except the vesicular structure, point to 
