JSjDJ On the Crystalline Structure of the Trap Dyles, ^-c. 2S7 



v.— On the Cri/sfallhie Structure of the Trap Dijkes in the Sienlte of 

 Amboor : icith an Enquiry into the Causes to which th's Peculiarity of 

 certain Igneous Rocks is due,— By Richard Baird Smith, Lieutenant, 

 Madras Engineers. 



The phenomena exhibited by the internal slructure of certain rocks, 

 which a large class of facts clearly prove to have been, more or less, 

 acted on by subterranean heat, have lately had the attention of observers 

 specially directed to them; and a wish has been expressed by an emi- 

 nent geologist,* that, since the study of these peculiarities has, as yet, 

 received but little attention, it would be therefore desirable, that informa- 

 tion should be collected relative to them, fur the purpose of enabling ns 

 to enquire into the causes of their production. Having lately been ena- 

 bled to examine the geology of the route from Madras to Bangalore, I 

 was especially interested in the results of an examination of the trap 

 dykes, in the sienite around the village of Amboor; and since in these 

 the phenomena, above alluded to, were strikingly exhibited, I have ven- 

 tured to throw together a few remarks deduced from these observations. 

 Since the field is one of comparative novelty, perhaps interest may be 

 excited, and the attention of those qualified to explore it, be directed 

 more particularly towards collecting the information relative to it, so 

 much desired. 



The underlaying rock of the plain on which the village of Amboor is 

 situated, is the sienite so universally met w4th throughout the range of 

 the ghauts. This sienite outcrops at various points in the plain, 

 and occasionally by its disintegration, gives rise to boulder-like masses, 

 which are found scattered around in great abundance. Mingled with 

 these, are weathered, and apparently water-worn, fragments of other 

 rocks, as of granite, gneiss, &c. varieties of trap, compact, and vesi- 

 cular felspar, quartz, and porphyries. The non-appearance of the parent 

 rocks, from which these fragments have been derived, in the neighbour- 

 hood, or actually in situ, proves that some transporting agent has 

 been acting upon them. This perhaps may have been the nullah which 

 intersects the plain, and seems to take its rise among the distant hills, 

 where, doubtless, the rocks above mentioned are largely to be met with, 

 since all belong to the primary series. These may have formed the bed 

 of the stream, and, thus being subject to continued abrasion, they would 

 readily yield to any increased force the current might acquire, from the 



* Mr. De la. Beche— " How to Observe.' 



