1840.] 



of tlie Great Basaltic District of India. 



tion* I have nothing to add ; but as it has been inferredf, that the " flat- 

 tened summits and long flat outline" of the low ranges connected with 

 this celebrated hill, and forming the eastern part of the great trap dis- 

 trict, are composed of basalt 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. 



The lower part of the hill 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 calcedon}'-, and, according to 

 Captain 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 situa- 

 tion 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 nodular 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 neighbourhood of every greenstone dike 

 or insulated mass of that rock, and under every layer of basaltic soil 

 in India, calcareous matter is deposUed, 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 



* Asiatic Researches, vol. xviii. 



t Geological Transactions, Second Series, vol. 4th, p. 410. 



$ Asiatic Eesearclies, vol. xviii., p. 199. 



