408 GEOLOGY OF THE OOMSTOCK LODE, 



such a supposition. In examining some fine instances of so-called perthite from the 

 original Canadian locality and from others on the Atlantic coast I have found the 

 reactions extremely variable, depending, as one cannot but suppose, on the relative 

 proportions of the two component feldspars which happened to be present in the little 

 fragment tested. In the andesite referred to, on the other hand, the reactions of the 

 feldspars were as regular as those obtained from different fragments of a single stand- 

 ard feldspar. The facts, therefore, do not appear to me to warrant the supposition 

 either that these crystals are mixtures of different feldspar species independently 

 crystallized or that they correspond in composition to some one of the unquestioned 

 varieties. They must rather be set down as isomorphous mixtures, in the sense in 

 which that term is to be understood in Tschermak's theory of feldspar-composition. 



