74 Dr. Mac Culloch on the Concretionary and 



the granular structure of granite, and the analogous rocks, being 

 of a crystalline nature, are here deserving of regard. It has beea 

 maintained that this structure has been the produce of watery 

 solution; since many geologists still chuse to consider granite 

 of aqueous origin, notwithstanding its analogy, nay, its identity, 

 in almost every circumstance of composition, texture, and acci- 

 dents, with the trap rocks, to which they admit an igneous one. 

 The argument, as far as its texture or structure is concerned, be- 

 longs properly to this place. 



Granting the greatest facilities to the preceding supposition, by 

 admitting the solution in water of earths noted for the extremely 

 limited degree in which they possess this property, and granting, 

 still further, that they were able, under these circumstances, to 

 enter into all the multifarious combinations which are to produce 

 quartz, feldspar, mica, hornblende, and many other minerals, it 

 remains to invent a new process in the chemistry of crystallization, 

 by which all these new combinations should have been in an instant 

 deposited together in a solid mass. If a successive deposition of 

 the different minerals be conceived, it is impossible to explain the 

 mutual interference which takes place among them, and which 

 characterises the crystalline granular structure. The imagination 

 that would produce such an effect from such causes, must not be 

 allowed to flit about vague generalities, but is bound to contemplate 

 steadily every minute circumstance implied in such a process. 



But nature and art both are ready to prove that this effect takes 

 place without difficulty from fusion. The glasses of our furnaces 

 separate into various mineral compounds on cooling. The same 

 results take place from the cooling of fused basalts, where the 

 previous combinations have all been dissolved by one general 

 fluidity. In the trap rocks, the granitic structure is common ; and 

 these, it is granted, are the products of fusion. The lavas of 

 volcanoes, if it could be necessary to insist on facts so well known, 

 are in a state of liquid fusion, in which every integrant earth is left 

 free to enter into such combinations as the infinite complication of 

 affinities may direct. If these are cooled suddenly, they are 

 arrested before they can enter into new compounds, and glass is 



