THE EVOLUTION OF IGNEOUS ROCKS. 161 



they have evidently been injected under great pressure. 

 They consist essentially of oligoclase, muscovite, biotite, 

 quartz, and subordinate microcline. Apart from the gneissic 

 structure, which is not constant, they differ from normal 

 granites, chiefly in their smaller proportion of potash-felspar 

 and quartz and in a peculiar rounded form of the crystals of 

 oligoclase. The micas, which crystallised earlier, are well 

 shaped, but the oligoclase, which came next, seems to have 

 been arrested in its growth before the crystals attained 

 their proper form. It resembles, indeed, the rounded 

 nucleus so frequently outlined in the felspars of normal 

 granites by zones of growth, but completed there by an 

 outer shell with crystal contours. What happened then to 

 interrupt the normal course of consolidation when the 

 oligoclase was but partially crystallised out ? The answer 

 seems to be that at this stage the residual liquid was in part 

 drained off and squeezed forward (in a southerly direction) 

 as a partial magma richer in potash and silica than the original 

 magma. It is now found consolidated in the form of peg- 

 matite. Tracing the gneiss southward, it is found to 

 change character. The oligoclase becomes less rounded 

 and forms a smaller proportion of the rock, the gneissic 

 structure is lost, and muscovite begins to predominate 

 largely over biotite. The rock is now permeated by strings 

 of coarse pegmatite, consisting essentially of microcline, 

 quartz, and muscovite, and this increases in bulk until a 

 massive fringe of pegmatite forms the southern border of 

 the gneiss area. Barrow concludes that in this area a 

 normal granite-magma was intruded in connection with the 

 great crust-movements which have affected the whole region, 

 and was injected under enormous pressure into every fissure 

 and line of weakness in the solid rocks. This was effected 

 concurrently with the progress of crystallisation in the 

 magma, and the liquid portion was thus able to enter 

 crevices too small to admit the already crystallised minerals, 

 and so to travel always in advance. The final residual 

 magma crystallised as pegmatite, and the gneiss and peg- 

 matite, are thus complementary products of the original 

 magma. Differentiation has here been brought about by 



