236 



/. H. OGILVIE 



is developed with large phenocrysts of olivine and no nepheline. 

 The olivine phenocrysts contain inclusions of magnetite, apatite, 

 biotite, plagioclase ; a few phenocrysts of twinned augite are occasion- 

 ally present. In this variety the groundmass contains intergrowths 

 of feldspars, and broad laths of both orthoclase and plagioclase. 

 The nepheline-bearing variety has a granitic texture, and lacks olivine. 



AUVERGNOSE III. 5. 4. 3 (AnDESITE) 



This rock is holocrystalline, dark-bluish gray, and contains many 

 basic segregations. It occupies the central part of the group and 

 lower levels of the surrounding hills whose tops are less basic. 



TABLE VII 

 Chemical Composition or Auvergnose 



In the following table the analyses are compared. The range 

 of silica is moderate, from 63 to 49 per cent. Alumina is generally 

 high and its range is not great. Iron and magnesia are uniformly 

 low and of slight range, with the exception of II, Auvergnose. Lime 

 is high and of great range ; soda is moderate and of little range ; potash 

 is moderate and of considerable range. Titanium is almost invariably 

 high; zirconia is entirely absent; baryta is high. 



