SYSTEM A TIC PETROGRAPHY 485 



Til. Rocks with alkali feldspar, without quartz or excess of silica, with 

 nephelite (haiiynite) or leucite. 



IV. Rocks with lime-soda feldspar, without nephelite or leucite. 



V. Rocks with lime-soda feldspar and nephelite or leucite. 



VI. Rocks without true feldspars, but with nephelite, leucite, or melilite. 



VII. Rocks without either feldspars or feldspathoid. 



Structure and geological age are applied by Zirkel under each 

 of the mineralogical groups, as follows : 



Granular rocks. 



(No distinctions by age.) 

 Porphyritic and glassy rocks. 



Pre-Tertiary. 



Tertiary and recent. 



The structural distinction is clearly in fact between (1) 

 granular and (2) non-granular, the range in structure within the 

 second division being by no means covered by the two terms 

 porphyritic and glassy. 



The use of age as a factor in classification of " porphyritic 

 and glassy rocks" while it is not applied to the granular rocks is 

 apparently more a recognition of the usage of the time, by which 

 a duplicate set of terms has been provided for effusive rocks, 

 than of any definite principle. The task of reconstructing the 

 nomenclature of the science is still one from which the systematic 

 petrographer shrinks. 



The group of the crystalline schists established by Zirkel is 

 not founded upon definitely stated principles, and is therefore 

 not a "systematic group. It is defined by enumeration of things 

 belonging in it or excluded from it, and must be treated as a 

 convenient expedient for purposes of rock description. But 

 although this is true the crystalline schist group of Zirkel is no 

 more unsystematic than the assemblages of other petrographers 

 given the same name. It is then germane to the present discus- 

 sion to state the actual course adopted in the Lehrbuch. 



Zirkel includes in his group of the crystalline schists, and as 

 its most important element, the pre-sedimentary gneisses, schists, 

 etc., which cannot be inferred from their attitude to other rocks to 

 be of igneous origin. Included with these are all rocks of the 



