1840.] Notes illustrative of the Geology of Southern India. 321 



hills in great abundance. It is unnecessary however to describe these 

 in detail here, as I have elsewhere done so at considerable length.* It 

 may therefore suffice to remark, that the trap, invariably, on being frac- 

 tured, exhibited cleavage planes, and separated into obtuse angled 

 rhomboids; that fissures traversed the dykes always at right angles to 

 the eooling planes or sides of the including sienite; that l:he direction 

 of cleavage was perpendicular to that of fissures ; and that the trap was 

 evidently altered at its planes of junction with the sienite. In attempt- 

 ing to trace the cause of these phenomena we are carried to the unex- 

 plored domain of modern science, for their development is dependant 

 on the action of molecular forces, whose origin and mode of operation 

 are alike enveloped in much mystery. Mathematical analysisf has been 

 successfully employed in demonstrating that, granting certain postu- 

 lates, the phenomena of crystallisation may result from the adjustment 

 of electrical forces; and observation tends apparently to confirm such a 

 theory, by shewing that crystals do actually result from the action of 

 forces derived from this source. But, we are still required to apply 

 these conclusions with diffidence, as their verification cannot yet be 

 considered as effected. The existence of electricity during the passage 

 of trap dykes from their original state of igneous fluidity to that in 

 which they now exist, may safely be concluded, from the experi- 

 mental truth, that variation of temperature is one of the most fruitful 

 sources of this agent ; and should it ever be satisfactorily shewn that 

 a subjection to electric agency confers on the particles of matter that 

 peculiar polarity, which disposes them to group themselves indeter- 

 minate figures, and under determinate laws, no element will be want- 

 ing for the application of such a principle of explanation to the present 

 case. 



The plain of the Carnatic is diversified by the occasional occurrence 

 of connected, as well as detached hills. A peculiar feature is to be 

 remarked in several of those in the immediate vicinity of Vellore. In - 

 dividual hills are observed to rise abruptly from the plain, like islands 

 in the midst of an ocean of sand, of the most perfectly conical shape, 

 and wanting only the crater, with its accompanying cloud of smoke, 

 to complete their resemblance to existing volcanoes. These hills are 

 usually at some distance from the connected ranges, and as an explana- 



* " On the crystalline structure of the trap dykes in the sienite of Amboor, &c. &c'." 

 Madras Journal of Science, July, 1839. 



t M. Mosotti sur les Forces 6[ui regissent la constitution interieures des corps 

 Wliewell's Hist, of Indue, Science, Sommeville Conn, of Phys, Science, 



