24 SCIENCE PROGRESS. 



plexity. A study of the practically infinite variety of rock- 

 types met with in nature shows that we obtain a closer 

 approximation to the truth by assuming two linear series 

 instead of one. Each series bridges over the interval 

 between the extreme acid and the ultrabasic rocks, and the 

 two lines diverge most widely in the middle. They may be 

 represented diagramatically by two arcs meeting at either 

 end. They are clearly traceable in either the volcanic or 

 the plutonic division ; the "dyke rocks," owing to the many 

 peculiar types which arise as local differentiation-products, 

 do not so readily fall into their places in this or any other 

 scheme. To fix ideas we will take the lavas, regarding 

 the rhyolites and the limburgites as the two extreme poles. 

 To the one series belong the pantellarites, trachytes, phono- 

 lites and leucitophyres, nephelinites and leucitites, nepheline 

 and leucite-basalts, and allied types ; while the other 

 includes the dacites, andesites and felspar-basalts. The 

 leading chemical distinction lies in the behaviour of the 

 alkalies, which in the former series increase in amount to a 

 maximum and then fall off towards the basic end, while in 

 the latter series they fall off steadily from the acid end to 

 the basic. The difference is thus most marked in the 

 intermediate members, the phonolites and allied rocks with 

 12 or 15 per cent, of alkalies presenting a strong contrast to 

 the pyroxene-andesites with at most 5 or 6 per cent. This 

 and some other differences in chemical composition naturally 

 express themselves in the mineralogical constitution of the 

 rocks, and most markedly so among those of niean silica- 

 percentage. In the former series we not only find rocks 

 composed mainly of alkali-felspars with little or no quartz, 

 but minerals of the felspathoid group (nepheline, leucite, 

 sodalite, etc.), often figure largely, while the characteristic 

 ferro-magnesian minerals are alkali-pyroxenes and alkali- 

 amphiboles in contra-distinction to the common augites and 

 hornblendes and the widely distributed rhombic pyroxenes 

 of the rocks of the second series. 



It has been remarked above that when a region of 

 igneous rocks belonging to one great system divides into 

 well-marked provinces, defined by their relations to axes of 



