94 J. P. ID DINGS 



of classification may be found, such that the two objects may be 

 accomplished at one and the same time. It is the purpose of 

 this paper to show that this is not consistent with the nature of 

 the case, and that a classification of rocks and the didactic treat- 

 ment of them cannot be based on the same method of procedure. 

 In order to do this it will be necessary to consider: (i) the 

 character of the constituents of individual rocks, both chemical 

 and mineral; (2) the results of a study of the chemical compo- 

 sition of all kinds of igneous rocks; (3) the occurrence of these 

 constituents in any rock as a result of processes of differentia- 

 tion, according to generally accepted theories ; (4) the chemi- 

 cal relations of series of rocks genetically connected; (5) the 

 nature of a rock-body or geological unit; (6) the nature of 

 rock-association, as in petrographical provinces. 



The character of the constituents of ig?ieous rocks both chemical a?id 

 mineral. — The same chemical elements occur as constituents of 

 nearly all igneous rocks. In the great majority of those ana- 

 lyzed oxygen, silicon, aluminium, iron, magnesium, calcium, 

 sodium, and potassium occur in measurable proportions; 

 while other elements, as phosphorus, titanium, manganese, 

 barium, strontium, chromium, nickel, cobalt, lithium, zirco- 

 nium occur in determinable amounts in many rocks, and in 

 traces in others. In many cases their presence has not been 

 sought. In numerous cases other elements are present in very 

 minute quantities as shown by the frequent occurrence of alia 

 nite containing the rare elements cerium, lanthanum, didimium. 1 

 Neglecting for the present elements commonly occurring in 

 very small amounts, the eight elements first named are those 

 which chiefly characterize igneous rocks. In very few cases one 

 or two of these may be absent or in traces only, but in almost 

 every case they are all present. Chemical differences among 

 these rocks consist in the proportions in which all of these ele- 

 ments exist in each case. Hence a classification of igneous rocks 



'Clarke, F. W., On the Relative Abundance of the Chemical Elements, Bull. U. 

 S. Geol. Survey, No. 78, 1891, p. 34. Also Bull. U. S. Geol. Survey, No. 148, 1897. 

 See also Zirkel, F., Lehrbuch d. Petrographie, 2d ed., 1893, Vol. I, p. 648. 



