258 J. Barrell — Relations of Subjacent Igneous 



saturation point is passed and crystallization begins on 

 particular foliation planes. Mica has mostly been left 

 behind in the magma, so the growth is principally by the 

 addition of quartz and feldspar. The continuation of 

 growth requires the pressing apart of the walls of each 

 lamina by the force of the growing crystals. Uneven- 

 ness of crystal growth keeps the whole in a suffi- 

 ciently porous condition to permit the passage of more 

 fluid. The expansion on many planes requires some mass 

 movement of the rock, resorption elsewhere, condensa- 

 tion, mashing, or crumpling. A slight accession of heat 

 and gases or pressure in one locality will turn crystalli- 

 zation into solution and resorption, permitting more 

 readily the expansion of other and adjacent parts by 

 intercrystallization. Rock flowage under such conditions 

 must be relatively easy, since the whole is within the 

 temperature range of crystallization and slight changes 

 of equilibrium will turn the balance from one side of the 

 equation to the other. The chief loss of energy is that 

 carried off by the solutions whose continuous passage is 

 needed to maintain the critical conditions. 



The different minerals are doubtless quite differently 

 susceptible to stress-differences. Although under nor- 

 mal conditions of great pressure but no stress-differences, 

 the micas crystallize before quartz and feldspar, if 

 stress-differences become great, then recrystallization 

 of mica appears to proceed more readily than that of 

 feldspar and quartz. The pressing apart of the magma 

 walls may thus result in a schistosity imposed while the 

 intercrystallization is still going forward. The process 

 of infiltration proceeds until any degree of granitization 

 has taken place. 



The process is not marked by the development of vein 

 structure ; rather each minute seam is like a pegmatite 

 or granite in texture. The crystallization is, therefore, 

 not so much against the walls as in between the walls. 

 As long as the solutions are passing, they are impelled 

 by the greater pressures of the magma below, and the 

 excess pressure over that of the surrounding country 

 rock at any place is expressed by the hydrostatic head 

 needed for overcoming the frictional resistances to flow. 

 This hydrostatic head is an essential factor, for it means 

 that the fluid can support the pressure of the growing 

 crystals and these do not need to resist porosity by 

 mutual pressure at their points of contact. A pegmatite 



