THE COMPOSITION OF KULA1TE 619 



latites, leucite-tephrites and, to include the tachylitic varieties 

 described in the original papers, hornblende-limburgites. These 

 rocks which, according to the principles of classification at 

 present in vogue, we must place in such diverse and distant 

 parts of our scheme, are all products of one center of eruption, 

 parts of the same undifferentiated magma, identical in chemical, 

 and in many points of mineralogical, composition. A rational 

 or natural scheme of classification should express their evident 

 close relationship, but on account of somewhat diverse mineral- 

 ogical composition, due to such extraneous and accidental 

 causes as conditions of cooling and the like, their mutual affini- 

 ties are to a large extent masked by the diverse names which 

 must be given them. 



We are brought face to face with the question, whether we 

 should hold to the present system, based on structure and 

 (largely qualitative) mineral composition; or whether we should 

 strive to base our classification primarily on the most funda- 

 mental character of igneous rocks — their chemical composition, 

 the quantitative mineral composition being a function and an 

 exponent of this. 



The choice of the latter would certainly seem to be justified 

 on sound theoretical grounds, and has been advocated by Pirs- 

 son, 1 Iddings, 2 Brogger, 3 Loewinson-Lessing, 4 and others. It is, 

 of course, also well known that Brogger holds that genetic rela- 

 tionships should find expression in classification ; in other words, 

 that the system of classification should be not a Linnean but a 

 natural one. But discussion of this and kindred topics would 

 carry us far beyond the limits of this paper. 



That any change will present difficulties of a practical as 

 well as of a theoretical nature is obvious. The aid of chemical 

 analysis, as well as of the microscope, must be invoked much 

 more often than in the past, though with increasing knowledge 



1 Pirsson, Amer. Jour. Sci., Vol. L, p. 478, 1895. 



2 Iddings, Jour. Geol., Vol. VI, p. 103, 1898. 



3 Brogger, Amer. Jour. Sci.,- Vol. IX, p. 458, 1900. 



4 Loewinson-Lessing, Compt. Rend., VII, Cong. Geol. Int., 1897, pp. 193-464. 



