202 



Notes on Geological Specimens from 



[Jul? 



From the top of the hill ofNugger above spoken of, numerous in- 

 sulated hills, and short ranges of a similar form, are seen to rise from 

 the granitic tract to the east and west, but they do not observe any 

 particular line of bearing, although the whole group seems to pass 

 in a direction from east to west, like the other basalt ranges of the 

 table land. From this hill to four or five miles north of Nirmul (a 

 large town nine miles north of the Godavery) as in almost all other 

 parts of the peninsula, is intersected by numerous greenstone dykes, 

 which generally run from N. by W. to S. by E. These dykes are of 

 great importance to the agriculture of the country, as the granitic soil 

 is extremely thin and poor, except in the valleys, where the. clay form- 

 ed by the decomposed felspar accumulates, and bears fine crops of 

 rice, for which water is collected in tanks, often in a great measure form- 

 ed of natural mounds of rounded or angular fragments of greenstone, 

 which is little subject to decomposition. At Jakrampilly, there is a 

 remarkable dyke of this kind, which can be traced for several miles by 

 a series of tanks on one side of it: it is also remarkable in exhibiting, 

 where it rises into a small hill near the village, the gradual transition 

 of the granite into the greenstone, and in the latter, having a tendency 

 to split into regular forms. When once a fissure, however small, is 

 formed, the rain washes a gradually increasing portion of lime and 

 other soluble parts of its surface into the interstices, until the masses 

 are separated, in which the alterations of temperature probably assist. 

 It is difficult to account for the manner in which the greenstone passes 

 into granite in this instance ; but it is evident, that it has been raised 

 by the granite above the continuation of the dyke at either end of the 

 hill. I have been more minute in the description of the hili of Nug- 

 ger, principally with the view of affording some information relative to 

 the distinction of the basalt ridges, which have burst through the gra- 

 nite of the Deccan, from the greenstone dykes,which are of such frequent 

 occurrence. The presence of olivine ; the soft wacke in which the 

 globular basalt is embedded; the less crystalline structure; the pas- 

 sage into amygdaloid containing calcedonies, zeolites, &c. and the gra- 

 nite in the neighbourhood of all the smaller masses of basalt, differing 

 little from that at a distance, may perhaps be sufficient to distinguish 

 these important rocks from each other. The separation of the differ- 

 ent ingredients of the granite into large crystals, and the insulated 

 masses of greenstone found in it near the dykes, prove, that the rock 

 had been softened by heat; but judging from the appearance and great 

 length of many of these dykes, I do not think that they were of con- 

 temporaneous formation with the rock through which they pass. Near 

 one of these, at Secunderabad, a smooth, wall-like dyke of white gra- 

 nite passes through the sienite. 



