the Eastern Portion of the Basaltic District of India. 545 



detected olivine in them*. The rock shows a tendency to separate into spheres composed of concentric 

 layers and into irregular prisms ; and the same structure in some degree occurs in the granite, sy- 

 enite, pegmatite, &c. of the whole of the south of India. The greenstone is exceedingly hard and 

 difficult to work, but it takes a most beautiful and durable polish, as in the magnificent mausoleums 

 of Golcondah, the tombs of Hyder Ali and Tippoo Sultan, at Seringapatam, and in many of the 

 sculptures of the Carnatic pagodasf. Where these dikes rise into hills, the summits only are 

 composed of the compact greenstone, which graduates below into the granite of the surrounding 

 country. Many of the veins of basalt in the passes of the Mysore and Neilgherry mountains differ 

 from these, in possessing the structure of the compact basalt of Bombay and other places in the 

 trap countries, and in branching into narrow veins, (often not an inch thick,) which traverse the 

 granite without mixing with it ; while the ordinary greenstone dikes of the Deckan are almost 

 always accompanied by separate nodules, of greater or less dimensions, insulated in the granitic 

 mass, the component parts of which appeared to me to be there, in most cases, arranged in larger 

 crystals, and to be more subject to decay, than in other places. I could not resist the inference, 

 that, at the time of the formation of these dikes, the granite was in a state approaching to fluidity; 

 although, as some of the narrow veins can be traced for many miles through the granite, they do 

 not appear to have been formed at the same time. 



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 gra- 

 nite. 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 scat- 

 tered 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 sonorous trap, and the upper of globular concentric basalt; the external lay- 

 ers 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. In some places the metallic 

 matter has the appearance of having been partially 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 contempo- 

 raneous formation, and not, as supposed by BerzeliusJ, a fragment of a pre-existing stone enve- 

 loped in the liquid matter. Small, but very characteristic 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 detected no calcareous 

 minerals in similar situations in these hills, although the rock is so impregnated with lime as to 



* See note to page 575. t It is familiarly known in India as " black granite." 



X Edinburgh Journal of Science, January 1839, 



