484 WHITMAN CROSS 



and is chosen in place of the primary arrangement of the first 

 edition because, as Zirkel points out, of inconsistencies and unnat- 

 ural associations which resulted, some of which have been men- 

 tioned in this review. Contact metamorphic rocks are treated 

 in connection with the igneous rocks which produced them. 

 Fragmental igneous rocks are placed with the elastics. 



In the systematic classification of igneous rocks Zirkel uses 

 the bases of arrangement in the following order : (1) mineral com- 

 position ; (2) structure; (3) age. The availability of chemical 

 composition, alone, or in expressed combination with mineral 

 constitution, is net discussed. The method of applying mineral 

 composition for the classification of igneous rocks is that com- 

 monly used. Concerning this Zirkel remarks : 



In a mineralogical arrangement of massive rocks the following considera- 

 tions are at present determinative : In the great majority of these rocks 

 feldspars and other silicates resembling feldspars (such as nephelite, leucite, 

 melilite) play the chief role, and therefore it is most natural to base the clas- 

 sification of such rocks upon the nature of these minerals, in accordance with 

 existing nomenclature. 1 



This procedure results in placing feldspar- or feldspathoid- 

 bearing rocks in one large group opposed to feldspar-free rocks. 

 Whatever the facts may be as to the relative quantitative impor- 

 tance of different minerals in igneous rocks, it is clearly arbitrary 

 to concede to any mineral the "principal role" where it is far 

 subordinate to others. The result is a qualitative expression of 

 mineral composition, bringing chemically unlike rocks together 

 in many divisions. 



In the descriptive portion of the Lehrbuch, igneous rocks are 

 grouped under seven heads : 



I. Rocks with alkali feldspar and quartz or excess of silica. 



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

 nephelite or leucite. 



J Fur die mineralogische Gruppirung der Massengesteine sind zur Zeit folgende 

 Erwagungen maassgebend : In der weitaus allergrossten Mehrzahl derselben spielen 

 Feldspathe und andere feldspathahnliche Silicate (wie Nephelin, Leucit, Melilith) 

 eine Hauptrolle und so scheint es am nattirlichsten, die Classification der hierher 

 gehorigen Gesteine auf die Natur dieser Mineralien zu begrunden, was zugleich der 

 bestehenden Nomenclatur entspricht. — Petrographie, Band I, p. 832. 



