m certain Trap Rocks. 79 



The onion stone of the Giant's Causeway, and some of the traps of Ayrshire, in 

 Scotland, and of the south of Ireland, (together with the spherical trap of Shiant 

 Island, mentioned by Dr. Maculloch,) consist of nodules, imbedded in a cement 

 of a texture and composition totally different from their own ; while each 

 nodule, on being fractured, breaks into successive spherical shells, or coats, 

 varying in hardness, and often in composition. 



If speculation may be ventured upon the foregoing observations, it would 

 seem to account for the phenomena to suppose, that the trap-dyke had been 

 evolved beneath the sea at a temperature of fluidity ; that in the violent agitation 

 produced by the formation and ascent of steam, &c. portions of the fluid mass 

 were projected upwards, became cooled in the water, and, falling again into the still 

 imperfectly molten bed, were by its motions gradually re-enveloped, and again 

 heated by contact nearly to its temperature. By these means (their texture being 

 similar) an imperfect union would take place between the nodule and its matrix. 

 When several of these nodules congregated, without intervening matter, they 

 would cohere with flat surfaces, as before described ; and when much more 

 highly heated, (having fallen from a greater height, and so sinking deeper in the 

 mass,) they would be again completely fused into the substance of the trap-rock, 

 and thus present the case above alluded to of the gradual obliteration of nodular 

 structure in some places. 



It is even not an improbable conjecture, that the most capriciously variegated 

 parts of this, and other similar traps and serpentines, may have been formed by 

 the soldering together of nodules of diverse matter, either projected from diffe- 

 rent depths, or broken from the adjacent rocks forming the walls of the dyke ; 

 and it is possible that even the singular contortions in the stratification of mica- 

 slate, &c. may have been produced by analogous means. 



It is worthy of note, that these developments are entirely due to the dissection 

 of the trap rock by the explosive force of gunpowder, but for which the discovery 

 had never been made. This seems, then, to place in the hand of the geologist a 

 new instrument for the prosecution of inquiries as to the intimate structure of 

 unstratified rocks ; — inquiries, which, should they reveal this nodular structure 

 as more general than it is now known to be, will be likely to add much to our 

 knowledge of the forces engaged in the production of rocks of igneous origin. 



