ROCK CLASSIFICATION 1 05 



rocks with variable proportions of the mineral constituents 

 are to be expected to be the rule rather than the exception 

 And when all igneous rocks are taken into consideration all pos- 

 sible combinations of mineral proportions may be anticipated. 



The chemical relations of series of rocks ge?ietically connected. — 

 The chemical characters of genetic series of rocks has been con- 

 sidered their most distinguishing and essential characteristic — 

 the one quality that established their consanguinity. 1 It might 

 be thought then that such a feature could be employed as a 

 means of classifying igneous rocks in groups corresponding to 

 genetic series. Careful consideration of the actual nature of 

 this chemical character will lead to the opposite conclusion. 



Certain genetic groups of rocks are distinguished by rela- 

 tively high alkalis, in one instance largely potash, in another 

 largely soda. Other genetic groups are characterized by rela- 

 tively low alkalis. But even in these series not every rock 

 contains the same high or low alkali ratio as the preponderating 

 varieties. In fact there are often members of the series which 

 are chemically quite different from the greater number of rocks 

 belonging to the series. In some genetic series it is the ratio 

 between the potash and soda that is characteristic, the total 

 alkalis being large in some rocks of the series and low in 

 others. But here again the ratio is never constant, and the varia- 

 tion may be quite large. 2 In no one genetic series yet described 

 has any single chemical character been found to be persistent 

 throughout the series. Nevertheless the transitions between the 

 rocks of the series are sufficiently pronounced to leave no reason- 

 able doubt that the rocks in question have been derived from 



1 Rosenbusch, H., Ueber die chemischen Beziehungen der Eruptivgesteine, 

 Min. u. petr. Mitth., Vienna 1889, Vol. II, pp. 144-178. 



Brogger, W. C, Die Mineralien der Syenitpegmatitgange der Sudnorwegischen 

 augit- und nephelinsyenite, Zeitscht. flir Kryst. u. Min., Leipzig, 1890, Vol. XVI. 



Iddings, J. P., Origin of Igneous Rocks, Bull. Phil. Soc. Washington, 1892, 

 Vol. XII, p. 135. 



2 Brogger, W. C, Die Eruptivgesteine des Kristianiagebietes. I. Die Gesteine 

 der Grorudit-Tinguait-Serie, Christiania, 1894, 165. 



Weed, W. H. and Pirsson, L. V., Geology of Castle Mountain Mining Dis- 

 trict, Montana, Bull. U. S. Geol. Survey, No. 139, 1896, p. 137. 



