the Eastern Portion of the Basaltic District of India. 559 



having the same stratified appearance as that of other parts of the formation, 

 it is necessary to mention, that they are formed of globular basalt such as 

 has been already described, or of basaltic columns of very regular forms, 

 which diverge from the centre of the hill and incline outwards at an angle of 

 45° with the horizon, or form a figured pavement on the flat summits. 



Origin of Minerals in Trap Rocks. 



Thelower partof thehill of Seetabuldee itself exhibits a tendency to columnar structure, caused 

 by horizontal and vertical partings, the sides of which are coated with thin plates of calcedony, and, 

 according to Capt. Jenkins*, of calc spar. These minerals are not confounded with the substance of 

 the basalt, and may be the result of infiltration or sublimation of siliceous and calcareous matters. 

 In no other situation did I meet with an example in which either of these processes would account 

 for the occurrence of the calcedonies, zeolites, calc-spar, &c. found in the amygdaloids and no- 

 dular basalt of India. Almost everywhere, calcareous spar is more rare than siliceous minerals, 

 which would not be the case were they derived from infiltration, if we are to interpret the past 

 by the present operations of nature. Wherever I have met with the basalt, and in the neighbour- 

 hood of every greenstone dike or insulated mass of that rock, and under every layer of basaltic 

 soil in India, calcareous matter is deposited, and has even occasionally a crystalline structure. 

 In the escarpments of the Mysore ghats, veins of basalt, not two inches thick, ramify through the 

 granite, and are coated with a compact layer of carbonate of lime. Voysey found the granite in 

 the neighbourhood of the basalt intermixed with calcareous matter, indications of which I have 

 myself seen, in the deposit of tuff on the summit of granitic logging-stones near greenstone. If, 

 then, we suppose infiltration to have deposited the calcedonies, agates, &c., &c., when chemical 

 action is presumed to have been more powerful than at present, a greater number of the cavities 

 would have been filled with lime than with such intractable substances, which is not the case. 

 These are also frequently intermixed with the basalt in a manner which could have been pro- 

 duced only by their being formed simultaneously. I have seen masses of calcedony, passing into 

 a black mineral not to be distinguished from the surrounding basalt. They also occur in the 

 compact nodules where no fluid could have had access, and even in the cavities of the Seetabuldee 

 basalt, lined by an impervious glassy coat. In this rock, likewise, calc spar occurs, penetrated by 

 needle-like crystals of the same substance, invested with a crust of the basalt and connected with 

 each side of the cavity, and resembling chiastolite in structure. It is indeed impossible to con- 

 ceive how many of the appearances presented by the agates, cornelians, or drusy cavities in cal- 

 cedony, partially or entirely filled by quartz crystals, or a central mass of calc spar, could be 

 formed in the way supposed, as the first coating of silex would effectually close out the further 

 access of the aqueous solution of that substance ; nor could layers so formed, separate into distinct 

 cavities, having both sides covered with quartz crystals, as they are sometimes seen to do. It is, 

 however, only by a careful study of the rocks themselves, that this can be fully understood. I 

 have selected a few specimens to show that the majority, if not all the minerals of the Indian trap 

 rocks, are not formed by infiltration. One of these, perhaps, deserves particular notice ; it is a 

 mass of calcedony, 8 or 10 inches in length and 6 or 8 in diameter, of a conical shape, and was 

 found imbedded with its apex downwards between the globular basalt, and impressed with the 

 irregularities of its surface. In another of these specimens the upper part is perfectly flat and 



* Asiatic Researches, vol. xviii., p. 199. 



