ON MINERAL VEINS. 391 



because the fact itself is extremely rare. It is an ex- 

 ception instead of a rule, and may be admitted without 

 involving the whole hypothesis. 



Since there must be an igneous theory to oppose an 

 aqueous one, in every thing, the other hypothesis 

 maintains that the contents of mineral veins were in- 

 jected from below as granite and trap have been. The 

 arguments for it rest partly on this very analogy, 

 partly on real or imaginary chemical facts relating to 

 the production of minerals by fusion, partly on some 

 mechanical appearances, and partly on the principle of 

 dilemma. If it be really a case of dilemma, the one 

 horn appears as fatal as the other, and there can be no 

 theory of mineral veins. 



The argument from the analogy of trap and granite 

 veins is one of those superficial resemblances, consist- 

 ing in words rather than ideas, which it is painful to 

 find in the writings of those who have been philo- 

 sophers in other things. It may be conceded that the 

 fissures have been produced by the same subterranean 

 changes which have displaced the strata; yet this ad- 

 mission does not involve a concession as to the rest of 

 the hypothesis. The presence of fragments of the 

 including rocks in the veins, which has also been used 

 as an argument, is a fact of no further value : it 

 proves the forcible displacement and fracture of the 

 strata, but nothing more. And this hypothesis has 

 forgotten, that, if the contents of these veins had been 

 injected in a state of fusion, the fragments so often 

 found in them should not have escaped this process. 

 I will not say, as has been objected, that clay could 

 not have existed in them on this principle ; because 

 the infiltration of water may decompose portions of 

 the veins, just as deep seated rocks are converted into 

 clay. As to the chemical arguments derived from the 



