THE GENESIS OF IGNEOUS ROCKS 307 



exist between igneous phenomena and earth movements, and 

 has at the same time been able to trace to a considerable extent 

 the progress of differentiation in igneous magmas. 



The distinguishing character of an igneous rock is that it 

 has consolidated from a state of fusion. If the solidification 

 is rapid, it results in the formation of a glassy rock which 

 does not differ in its essential nature from the fluid mass from 

 which it has originated. There has been no break in continuity, 

 but the viscosity has increased to such an extent that the magma 

 may for practical purposes be regarded as a solid. 



If, on the other hand, the process of solidification has been 

 sufficiently prolonged, the molten magma crystallises out into 

 silicates and other minerals. Simultaneously with the crystal- 

 lisation, or on a decrease of pressure at a previous period, 

 a number of volatile substances, which also formed part of the 

 magma, separate from it and make their way through the sur- 

 rounding rocks towards the surface. The most important of 

 these "volatile fluxes" or " mineralisers," as they are termed 

 by different writers, is water, which even at the high tempera- 

 ture of the magma retains, if the pressure be sufficiently great, 

 many of the physical properties that characterise it at ordinary 

 temperatures, but is endowed at the same time with far greater 

 chemical activity. Volatile fluxes are usually present in 

 greatest amount in acid magmas, especially those under great 

 pressure at a considerable depth. 



It was formerly assumed that the chemical constituents of 

 the silicates were present in the magma in the form of simple 

 oxides, and that these combined to form minerals when the 

 rock consolidated. But we can only explain the manner in 

 which the oxides are distributed in the minerals as they suc- 

 cessively crystallise on the hypothesis that the former were 

 already in a state of combination in the magma. 



" Thus the bulk-analysis of an ordinary granite shows more 

 alumina than is required to make felspars with the alkalies 

 and lime present, and this excess is contained in micas or 

 aluminous hornblende, minerals of variable composition. But 

 the last-named minerals, with others of minor importance, have 

 crystallised before the felspar. By their abstraction the com- 

 position of the remaining magma was accurately adjusted, so 

 that the molecules of alumina equalled the sum of the molecules 

 of potash, soda, and (remaining) lime. In other words, it was 

 reduced accurately to the composition of a mixture of felspars 



