256 J. Barrell — Relations of Subjacent Igneous 



It is in this way, by the movement of solutions along 

 foliation planes, rather than by a mere recrystallization in 

 place, that marked lamellar segregation of minerals in 

 gneisses is most readily explained. Let a massive gran- 

 ite yield to lateral pressure by granulation or recrystalli- 

 zation : the minerals are broken down and reconstructed, 

 but in this reconstruction laminae of quartz and feldspar 

 tend to alternate with those of mica. Mere molecular 

 diffusion and crystallinic attraction would tend to build up 

 individual minerals elongated in the plane at right angles 

 to compression, but would tend to reconstruct the miner- 

 als about each dominating nucleus without extending one 

 kind of mineral into continuous sheets. This kind of 

 massive gneiss is fairly connnon in those types where 

 granulation has exceeded recrystallization as the agent of 

 deformation, in other words, where regional compression 

 has been strong and crystallizing solutions scanty. 



Where, however, the solvents are more abundant, move- 

 ment would take place readily along the foliation direc- 

 tion, very slowly across it, the biotite would be pro- 

 gressively concentrated into layers, the quartz and feld- 

 spar would tend to be carried farther along the planes 

 and have the relations of intercrystallized laminae. If 

 the solvents rise far and are concentrated into thicker 

 sheets, it is a form of pegmatization carried on not as a 

 result of a primary crystallization of magma, but as a 

 result of rock-mashing in the presence of rising solutions. 

 Pegmatization, as Van Hise has noted, may occur in 

 association with formations which show no relationship 

 to igneous intrusion. In granite gneisses which show a 

 pegmatitic texture, as in the Hoadley Point, Connecticut, 

 gneiss, the bands of quartz and porphyritic feldspar may 

 be an inch across and separated by continuous sheets 

 of biotite. 



In gneisses whose lamellar character is due to mashing 

 and recrystallization, the emphasis is put here not merely 

 upon the presence of crystallizers, but upon their passage. 

 The crystallizers are to be interioreted as the rising 

 emanations from deeper seated sources. 



Development of Lit-par-Lit Structure by Force of Crystal- 

 lization. 



Lit-par-lit injection is that form of magmatic intrusion 

 which takes place where the magma has soaked into a 

 highly foliated roof rock and has resulted in all grada- 



