1810.] 



of the Gnat Basaltic District of India, 



67 



Several small basaltic hills are insulated in the granitic platform in 

 the line of route between Hyderabad and Nirmul, and they are based on 

 decaying granite. Their flat summits and steep sides correspond with \ 

 the hills of the great trap district ; and the general line of bearing, of 

 the broken ranges of which they appear to form a part, does not differ 

 much from that of the basaltic mountains to the north, or from the 

 greenstone dikes ; but many hills are scattered over the plains to the 

 east and west, and cross this line of route in every direction. The 

 lower part of the hills is composed of thin tables or laminae of a sonor- 

 ous trap, and the upper of globular concentric basalt ; the external 

 layers of which are extremely friable, generally grayish, or soft and 

 soapy to the feel, and are of a greenish tint, except where the stone is 

 much loaded with ferruginous grains of a reddish brown colour. la 

 some places the metallic matter has the appearance of having been par- 

 tially smelted, and is of a fine red hue. 



The nuclei of the " concentric basalt," which are exceedingly tough, 

 and resist decomposition powerfully, are of a deep black colour, and 

 contain large crystals of olivine, and small kernels of calcedony. The 

 first of these minerals is not found in the soft external coats, yet it is so 

 closely united with the substance of the rock as not to admit of a doubt 

 of its being of contemporaneous formation, and not, as supposed by 

 Berzelius*, a fragment of a pre-existing stone enveloped in the liquid 

 matter. Small, but very characteiistic specimens of calcedony are of 

 more frequent occurrence in the softer portions of the rock, especially 

 between the concentric nodules ; but they are intimately mixed with their 

 substance. It is remarkable, that I detached no calcareous minerals 

 in similar situations in these hills, although the ro' k is so imipregnated 

 with lime as to have led to an extensive deposit of calc-tuff (Kunkur), 

 at present forming along its base, and projecting sometimes half a foot 

 from between the partings of the basaltic tables. — " The presence of 

 olivine, the soft wacke in which the globular basalt is imbedded, the less 

 crystalline structure, the passage into a porous amygdaloid, containing 

 calcedonies, zeolites, &c., and the granite in the neighbourhood of all 

 the smaller masses differing little from that at a distance," may perhaps be 

 sufficient to distinguish these insulated basaltic hills from the greenstonet. 



On approaching the Godavery, the granite in some degree changes its 

 appearance, containing large and beautiful crystals of red felspar, oc- 

 casionally imbedded in veins of transparent quartz, clouded with spots 



♦ Edinburgh Journal of Science, January 1839. 



+ Journal of the Asiatic yociety of Bengal, February 1836, p. 105. 



