134 Dr. H. Warth — Chemical Classification of Igneous Rocks. 



My present object is to employ a moderate number of divisions, 

 and for this purpose 16 groups appear to answer best. The com- 

 bination of similar bases has enabled me to simplify the system. 

 The individual bases maintain generally a correct proportion, and 

 when they do not the bases of a pair may easily be understood to 

 replace each other (see Table II). 



The separate table (Table I) shows the average compositions 

 of the 16 groups which are here used. There is added for each 

 group the name of a typical rock which occurs in the group. To 

 facilitate the discussion of the table I also add a Diagram (p. 133) 

 which requires some explanation. The vertical bands of the diagram 

 are of such widths that they represent by the scale the average 

 percentage of the seven basic oxides and of water. 



If a line be drawn from point of the diagram inclined at 45°, 

 and if we then measure from the intersection of that line with 

 the right margin of each band along that margin the sums of 

 the bases AL O3, ALOs + FesOs, AL O3 + Fea O3 + Fe 0, etc., etc., 

 we obtain a straight horizontal line which stands for the average 

 of 500 rocks. If we apply the corresponding numbers for any one 

 of the rock groups we obtain the curves shown on the diagram. 

 The acid rocks have descending curves, the basic rocks have 

 ascending ones. The descent of a portion of any curve implies 

 that the proportion of the respective bases is below the average, and 

 vice versa. Horizontal portions occur in a curve whenever the 

 respective components are equal to the average of the 500 rocks. 



In order to illustrate further the degree of proximity within the 

 groups of rocks I herewith give an example of an entire group 

 (Table II). This group No. 9, 6 a a a, has a mean total deviation 

 of 9"4, which is very close to the average of all the groups and will 

 therefore give a fair idea. Other more acid groups would show 

 a greater proximity, whilst the most basic groups would be most 

 divergent, as may be seen on the above-mentioned Table I. 



Scheme of Division into 16 Eock-Groups. 

 I 24-S 



Group 



Although the above system is only based upon 500 selected rooks, 

 any number of others may be incorporated by following the same 

 standard averages. In the separate Table I, the averages are given 

 which served for the establishment of the 16 groups. These same 



