34 Dr. Mac Culloch on the Origin, Materials, 



Thus analogy, resemblance, and experiment, confirm that opinion 

 respecting tlie trap rocks, which would be inferred from the pe- 

 culiarities of their chemical constitution ; and thus also they 

 confirm the conclusions that may be drawn from their peculiar 

 disposition, and from the nature of their connexion with the 

 various conterminous rocks among which they are found. 



It is hut a step from the trap rocks to granite ; and if the 

 identity of specimens is not always so perfect, or the resem- 

 blance so general and extensive between these and the volcanic 

 rocks, the analogical reasoning is quite as unexceptionable. I 

 «howed, in a former paper in this Journal, that many rocks, 

 forming integrant portions of a granite mass, are undistinguish- 

 able from many of the trap rocks, and that among these there 

 are many that resemble the productions of volcanoes. Here, 

 then, is an identity, even between granites and volcanic rocks ; 

 and here, also, what is true respecting the origin of one part of 

 the mass, must be true respecting the whole. If that inference 

 appears to be drawn closer than the circumstances seem to war- 

 rant, we may carry it through the intermediate stage of trap ; 

 and having thus proved the identity of this rock with the volcanic 

 products on the one hand, and with the granite on the other, apply 

 a common mathematical axiom to the conclusion. 



If it be said that volcanoes do not produce granite, it must 

 still be recollected that they produce compounds of an analogous 

 nature in every respect. It was also shewn, in the paper refer- 

 red to, that the trap rocks often assumed the character of per- 

 fect granite ; so that, by this intermediate step, the several pro- 

 ducts which are most distant are again associated. Even ad- 

 mitting that the volcanic rocks stood exclusively at one extremity 

 of a scale of chemical compounds, and the granites at the other, 

 the trap rocks containing examples of both, form the common 

 link by which they are united. This view of the chemical origin 

 of granite is confirmed by the same set of appearances which 

 confirm it in the case of the trap family, and which have been 

 sufficiently described by various writers. 



It is not difficult to assign probable reasons for the differ- 



