Dr. Mac CuUoch on Mineral Veins, 197 



solution. It has also been said by the supporters of this hypo- 

 thesis, that the absence of the solvent water from the veins is 

 a proof that their contents were not deposited from water. It 

 assuredly does not prove that ; while, as it respects the igneous 

 theory, it is merely an argument from dilemma, that proves 

 nothing in its favour, if it be not truly a case of dilemma. In 

 having recourse to this species of reasoning, the first step is to 

 establish the necessity of the alternative. 



Another imaginary chemical argument has been derived from 

 the mutual impressions of co-existent crystals in the veins. This 

 is a view founded on the nature of granite, and other rocks 

 crystallized from fusion ; but it is an analogy which has been 

 abused, no less in this case, than in that which relates to the 

 nodules of the amygdaloids. The mutual impression of quartz, 

 or of chalcedony, and calcareous spar, does occur in these, 

 from successive infiltration and crystallization; and, according 

 to the order in which these substances are deposited, either 

 may impress the other, as I have fully shewn in my work oa 

 the Western Isles. It is perfectly consistent with this to imar 

 gine, that any number of minerals admitted, at distinct inter- 

 vals, into cavities, should present the same appearances ; and 

 that, in modes much more complicated than could happen from 

 any simultaneous crystallization from an uniform fluid of fusion. 

 But, in truth, though the inconceivable chemical agencies re- 

 quired to separate all the minerals that are found in a com- 

 pound rock, have been made almost a subject of ridicule against 

 the supporters of aqueous theories of recks, it would be diffi- 

 cult to imagine any process more difficult than that which 

 should crystallize all the variety of earthy and metallic minerals 

 that are sometimes found together in veins, from an uniform 

 fluid of fusion. Chemists who will bestow a moment's con- 

 sideration on this subject, will see without difficulty what it is 

 unnecessary to detail here. 



Some farther arguments, as much mechanical as chemical, 

 have also been adduced in favour of the igneous hypothesis. 

 It has been said, as an argument from dilemma, that, on th& 



