ELISHA MITCHELL SCIENTIFIC SOCIETY. 83 



The direction taken by the mica crystals is not without in- 

 terest. As a rule the plane of crystallization, parallel to lamina? 

 of the mica, is more or less inclined to the line of strike, being 

 frequently perpendicular to it, so that the mica ou being uncov- 

 ered resembles a pile of thick planks laid flat on the sole of the 

 level. I do not recall an instance of a contrary arrangement, i. e., 

 of a parallelism between the plane of lamination and the line of 

 strike. The tendency is strongly the other way. 



An interesting question here is whether the mica, feldspar, 

 quartz, garnet, etc., existed as such within the vein, and had only 

 to segregate themselves by crystallization, or whether they are to 

 be regarded as forming within the liquid mass highly complex 

 silicates, which crystallized according to the chemical affinity of 

 their constituents under the existing circumstances. According 

 to the first view, the mica probably existed as H 4 K 2 (Al 2 ) Si 6 24 , 



the feldspar as K 2 (Al 2 ) Si 6 16 , the garnet as R 3 (R 2 ) Si 3 12 , where 

 R = Ca, or Fe, or Mg, and (R 2 ) = (Fe 2 ) or(Al 2 ), and the quartz 

 as SiO 2 . They existed as such, and had only to crystallize to 

 become visible. 



According to the second view, the potash, alumina, lime, mag- 

 nesia, iron and silica were all in a state of aqueo-igneous fusion 

 together; some of the potash and alumina lay hold of the requi- 

 site amount of silica and became mica ; another portion of the 

 potash and alumina and silica formed feldspar, etc.; the portion 

 of silica not needed for these compounds finally crystallized as 

 quartz. In neither case could crystallization occur until the 

 critical point (congelation point) for each substance was reached. 



The various chemical elements in the vein matter would at the 

 moment of crystallization have affinities influenced by the tem- 

 perature, pressure, etc., and these affinities might or might not 

 be the same as at ordinary temperature and pressure. That a 

 high heat does influence chemical combination is a fact too well 

 known to be more than re-stated. Thus it is well known that at 

 a glowing heat oxygen has a greater affinity for carbon than for 

 either hydrogen or iron, strongly as it tends to combine with 

 these two elements. The chemical affinities existing between a 



