250 SCIENCE PROGRESS 



and aegirine-syenite) ; or (3) by the characteristic mode of 

 crystallisation of a constituent (as in the laurvikite and 

 laurdalite of Brogger, peculiar types of syenite and nepheline- 

 syenite, in which the crystals of the dominant felspar are 

 rhomb-shaped). Although names, founded on mineralogical 

 variation in constituents other than the felspars, are of little 

 service in classifying rock-types, they are often useful for 

 descriptive purposes. They form no part, however, of the 

 system of classification now undergoing discussion. 



I propose now to show by a selection of analyses the 

 chemical correlation of the main types in each series. This can 

 be done graphically by Iddings' 1 method, in which the mole- 

 cular proportions of the silica are represented by abscissae 

 and those of the bases by ordinates, or by Brogger's 2 modi- 

 fication of Michel-Levy's 3 method, in which the molecular 

 ratios are plotted so as to give a characteristic picture of the 

 rock-type. In Brogger's modification the molecular ratios are 

 plotted on axes radiating from a central point O : four of these 

 are formed by a vertical and a horizontal line intersecting at O, 

 and four by lines forming an angle of 120 with the vertical 

 and horizontal lines and also intersecting at O (see fig. 1). 

 On the horizontal line the molecular proportion of the silica 

 is laid off, 4 half to the right and half to the left. On the vertical 

 line the alumina is laid off downward, the lime upward. The 

 ratios for the iron (FeO) and magnesia are laid off on 

 the oblique axes above, and the soda and potash below the 

 horizontal line (see fig. 1). By joining the points thus obtained, 

 a picture of the rock is produced, the shape of which at once 

 shows the chemical character of the rock. 



An example will make the matter perfectly clear. Suppose 

 a diorite to have the chemical composition represented by the 

 figures in column 1 of the following table, then the molecular 

 proportions of the silica and the bases will be as shown in 

 column 2, and the lengths to be laid off in millimetres will 

 be as shown in column 3. 



1 J. P. Iddings, "The Mineral Composition and Geological Occurrence of 

 certain Igneous Rocks in the Yellowstone National Park," Bull. Phil, Soc. 

 Washington, vol. xi. 1890, pp. 191-220. 



8 Brogger, Das Ganggefolge des Laurdalits, loc. cit. Christiania, 1898, p. 257. 



3 Michel-Levy, Bull. d. serv. de la carte geol. de la Fra?ice, No. 57, vol. ix. 

 (1897). 



4 A convenient scale is 100 mm. to the unit. 



