390 



ALFRED HARKER 



come out most clearly when exhibited graphically in a diagram 

 such as was first used by Professor Iddings. 1 



The diagram is easily constructed from the analyses of the 

 rocks (Fig. i). Two rectangular axes, OX and OY, are drawn, 

 horizontally and vertically, each of length equal to ioo parts of 

 some convenient scale. We cut off the abscissa 

 OM to represent the silica-percentage of any 

 one of the rocks, and erect ordinates MP, MQ, 

 etc., to represent the corresponding percent- 

 ages of the other oxides, which we 

 mveniently speak of as the 

 This is done for each of the 

 Then joining P P' . . . . ., 

 etc., we have a line 

 which represents the 

 variation of one par- 

 ticular base in this 

 rock-series, and simi- 

 larly for the other bases. 

 Iddings converts the 

 figures of the analyses into molecular proportions before express- 

 ing them graphically ; but for our present purposes nothing 

 would be gained by doing this, and we may follow the simpler 

 method. 



The lines PP' , etc., QQ' , etc., as thus laid 



down, are broken or zigzag lines. We find, however, that in a 

 rock-series in the strict sense of Brogger the departures from 

 regularity are not great. Minor irregularities may arise from 

 the variation found among different specimens of the same rock- 

 type, or from errors in the analyses. To obtain a clearer picture 

 of the essential variations characteristic of the series it will be 

 legitimate to smooth the lines, i. e., to convert them into flowing 

 curves passing as nearly as may be through the proper points. 

 The diagram then affords a graphic representation of the chemi- 

 cal characteristics of the given rock-series. We may use it, for 

 1 Origin of Igneous Rocks, Bull. Phil. Soc. Wash. (1892), Vol. XII, pp. 89-214. 



Fig. 1. 



