A. Holmes — Classification of Igneous Rocks. 129 



felspathoid, and olivine, it really constitutes a classification on three 

 co-ordinates. To make quantitative' divisions the ratios of quartz 

 to felspar, felspathoid to felspar, and of olivine to felspar or mafic 

 minerals, could he employed. In particular it should be pointed out 

 that in the table of oversaturated rocks, quartz-syenites have been 

 squeezed out by the granites, and only consideration for space has 

 deterred the writer from inserting a second sheet to include rocks 

 with small amounts of quartz (on which such rocks as quartz-gabbro 

 would then have properly appeared). The fourth and fifth co- 

 ordinates, printed as second and third dimensions, are quantitative, 

 and depend respectively on the ratios of albite to anorthite (expressed 

 by percentages of anorthite), and of orthoclase (including other 

 potash minerals suitably weighted) to albite (including other felsic 

 soda minerals suitably weighted). A remaining feature, unexpressed 

 in the tables, is the ratio of felsic to mafic minerals. The classification 

 and nomenclature arising from this ratio have already been described ; 

 and in practice the ratio constitutes a sixth co-ordinate that may 

 often be of great service as a further means of subdivision. The 

 seventh and last co-ordinate is textural. 



Vogt (see Harker, p. 373) has pointed out that in the peridotites 

 the atomic ratio of magnesium to iron gradually increases as the 

 amount of alumina decreases. Dr. Prior has also used the ratio of 

 MgO/FeO with excellent results in a recent and illuminating 

 classification of meteorites {Min. Mag., p. 42, 1916). It may be 

 worthy of notice that in the mineralogical classification of this 

 paper, the same ratio increases from the north-east corner of each 

 sheet to the south-west corner, for rocks having a normal "colour 

 ratio " or felsic/mafic ratio. Indeed, for all such rocks (as far as 

 •can be tested by "average" analyses) the chemical variation in any 

 direction is approximately regular. 



Glassy rocks are necessarily incapable of treatment, as indeed 

 they must be by any system of classification by minerals. Similarly, 

 most , altered rocks, whether they be altered by pneumatoly tic or 

 other processes arising from the consolidation of their parent magma, 

 or by weathering processes, must also be excluded. A classification 

 appropriate to express their characters would be based on the 

 processes by which they have been . altered, and the mineral and 

 structural changes whereby their new features have been developed. 



Eeferences. 

 Cross, Iddings, Pirsson, & Washington. " Quantitative Classification of 



Igneous Eocks " : Journ. GeoL, x, p. 555, 1902. In book form, 1903. 

 " Modifications of the Quantitative System" : Journ. GeoL, xx, p. 550, 1912. 

 Cross, W. " The Natural Classification of Igneous Eocks " : Q.J.G.S., lxvi, 



p. 470, 1910. 

 "The Use of Symbols in expressing the Quantitative Classification" : 



Journ. GeoL, xx, p. 758, 1912. 

 " Problems of Petrographic Classification " : Journ. GeoL, xxii, p. 791, 1914. 

 Evans, J. W. " The Quantitative Classification of Igneous Eocks " : Science 



Progress, No. 2, October, p. 258, 1906. 

 Harker, A. The Natural History of Igneous Bocks, ch. xv, 1909. 

 Hatch, F. H. " The Classification of the Plutonic Eocks " : Science Progress, . 



No. 10, October, 1908. 

 Text Book of Petrology, vol. i, 1916. 



DECADE VI. — VOL. IV. — NO. III. 9 



