1339] 



Trap Dijhes in the Sienite of Aniboor. 



299 



nature like the present, if we find that in our group of facts there is 

 anj^one circumstance in which they all without exception agree, that cir- 

 cumstance may be the cause in question, or at least a collateral eiFect of 

 the same cause, if there be but one such point of agreement, this possi- 

 bility becomes a certainty, and if on th;^ other hand, there are more than 

 one, they may be concurrent causes*." 



Looking then to the group here assembled, we find that their subjec- 

 tion to a variation of temperature is the single point, in which they all 

 agree ; this, therefore, according to the above rnle, given by Sir John 

 Herschel, must be, either the cause itself, or intimately connected with 

 the cause. We now proceed to examine this point, and in order that the 

 results may be more clearly exhibited, one instance will be taken, and 

 the various circumstances connected with it examined and discussed. 

 Selecting, then, for this purpose, the trap dykes — we know% from gene- 

 rally acknowledged geological theories, that the trap of which they are 

 formed, was originally ejected from the interior of the earth, at a tem- 

 perature so high PS to be kept by it in a state of fluidity similar to that 

 of the lava currents of existing volcinioes. Let us therefore suppose 

 we have before us a formation of this kind, an intensely heated and fused 

 mass, included between two walls formed by the fissured rock. From 

 the action of that law% by which bodies tend to an equalization of tem- 

 perature, the moment contact was made betw^een the heated trap and its 

 including rock, an abstraciion of caloric from the former by the latter 

 would take place, and as the heat travels slowly from particle to particle 

 of the including rock, to distances dependant on its conducting power, 

 a continual demand is made on the trap, so that we may conceive two 

 currents to be established flowing from each side of the dyke through 

 the adjoining rock forming its walls. Now it is a singular fact, and one 

 by which this enquiry is most materially advanced, that, under circum^ 

 stances precisely similar to those under which the trap is here described 

 as being, a combination is formed, by which large quantities of electri- 

 city are developed, and this development seems to be an effect of the 

 variation of temperature. Hence, then, the idea that the cause we are in 

 search of is this variation, becomes merged into the more definite one — • 

 that electricity in the active and operative agent, to which we are to 

 attribute the phenomena whose production we are investigating. In 

 electricity we have all the essential requisites for a cause by which na- 

 tural phenomena may be explained, we know of its existence, of its be- 



* Sir J. Herschel's Discourse on Natural Philosophy. 



