350 SCIENCE PROGRESS 



mica within the granite are " planes of flow or sliding surfaces," 

 the primary arrangement of this mica having been due to viscous 

 flow near and parallel to a bounding surface. " On the appli- 

 cation of pressure, shearing evidently commenced along lines 

 determined by the mica plates, and was accompanied by the 

 crushing of the quartz mosaics, while the felspar as a rule 

 yielded but little : the crushed material set flowing moved in 

 stream-lines round the immersed felspar, which remains as the 

 ' eyes ' in the ' flaser ' structure." 



The foregoing description by Prof. Sollas of a granite in 

 Co. Wicklow applies to most cases of foliated granites con- 

 taining micaceous bands ; but the microscopic evidence of 

 crushing is by no means always so apparent. The comminu- 

 tion of the crystals of quartz, their flowing round the felspars, 

 the consequent " eye-structure," and the wavy banding of the 

 mica flakes, are all paralleled in cases of undoubted igneous 

 flow. 2 Indeed, as a molten rock becomes viscous and begins 

 to crystallise, it flows — as it probably did from the beginning — 

 under the influence of some earth-pressure far away behind it. 

 The same pressure that rears the dome of superincumbent 

 sediments, and permits of the entry of the molten magma, 

 forces that magma into every crevice long after crystallisation 

 has begun. Even the fracture of crystals and the production 

 of the well-known optical " strain-shadows " need not in all 

 cases be assigned to a period of crush subsequent to that of 

 consolidation. 



The fact that igneous rocks move forward, and are operative 

 as true intrusive masses, after many of their constituents have 

 appeared as prominent crystals, is proved by many interesting 

 features in the zones of contact. Barrow 3 has described the 

 filtering out of oligoclase crystals from a granite, and their 

 accumulation in the main mass of the rock, because the 

 veins that had opened on the margin were too narrow 

 to admit them. The ultimate intrusive material in these 

 veins thus differs, through physical causes, from the granite 

 from which it is derived. This is, of course, only a striking 



1 Scrope long ago noticed this sliding action among the constituents of gneiss 

 {Considerations on Volcanos, 1825, p. 234). 



3 Cole, " Metamorphic Rocks in E. Tyrone, etc.," Trans. R. Irish Acad., 

 vol. xxxi. (1900) p. 440. 



3 " On certain Gneisses with round-grained Oligoclase," Geol. Mag. 1892, 

 p. 65. 



