1839] 



Trap Dylces in the Sienite of Jmhoor, 



3or 



eles, of a body, and the arraDgement of these particles in their definite 

 forms, a point of so important a bearing on the nature of this enquiry. 



At a future time, I trust to be able to add one more link to the chain 

 of reasoning, by which the preceding results have been arrived at, by 

 giving the details of a series of experiments on the efi'ects of electrical 

 currents passed through bodies in a state of fusion, from which I antici" 

 pate additional confirmation to the foregoing view of the cause to which 

 the crystalline structure of trap, basalt, and other rocks of igneous origin 

 is due, as well as that of certain sedimentary deposits, acted on, and al" 

 tered, by heat of variable intensities. 



Perhaps it may now be advisable to cast a retrospective glance at the 

 ground over which we have passed, and to exhibit, by a short recapitula- 

 tion, the steps of our enquiry, so as to enable the memory lo retain the 

 leading points the better, by divesting them of the intervening details. 

 The object to be attained was to determine, as far as we could, the cans© 

 of the crystalline structure of trap and other igneous rocks. To this 

 end analogous phenomena were classified, and discussed, their general 

 bearing in regard to each other exhibited, and the bond of union, the 

 point in which they are all agreed, was, in obedience to the laws of in- 

 duction, considered as either the cause itself, or intimately connected 

 with the cause of the phenomena in question. A law, based on the facts 

 reviewed, was there expressed, at first limated in its application, but sub- 

 sequently extended, by the discussion of other phenomena, till, com- 

 mencing with masses in a state of intense heat, it gradually descended 

 till it was found to apply to frozen water. The law of the connection 

 of crystalline forms with variation of temperature, as exhibited in the 

 7 classes, was then more minutely examined, and, by its extended discus- 

 sion, variation of temperature was found to be a fruitful source of one 

 of the most active secondary agents in the natural world, viz. electricity. 

 The peculiar case of trap dykes was chosen to illustrate the action of 

 this power, into which the original one was now resolved. Its develop- 

 ment, its efficiency, and its mode of action, were successively examined in 

 detail, and analogies were brought to bear on these different points . The 

 field of inquiry is yet far from being exhausted, the dependance of cry- 

 stalline form on chemical composition as exhibited in the laws of iso- 

 morphism, the resolution of chemical action into the preponderance of 

 different electrical forces, as lately proved by Dr. Faraday, the connection 

 subsisting between heat, light and electricity, are all paths open to 

 and inviting research, full of interest, and promising most rich and fruit- 

 ful returns. 



