Invasion to Regional Metamorphism. 259 



vein may therefore hold a porous texture and yet be 

 solid during the time of growth, in much the same way as 

 a porous sandstone may be strong and yet serve as a 

 passage for circulating waters. The difference, however, 

 is that the hydrostatic head of the meteoric waters is 

 less than that of the sandstone, with the result that the 

 recrystallization of the sandstone eliminates the porosity 

 and develops a quartzite. In the pegmatite, on the 

 contrary, the porosity would be maintained as long as the 

 country rock could yield before an excess of hydrostatic 

 head from the magmatic emanations. The last stages in 

 the development of the crystallization would be a final 

 elimination of the porosity. It would seem that this 

 is a rather fundamental theoretic principle whose 

 existence it is necessary to postulate in order to obtain a 

 workable understanding of the phenomena of regional 

 granitization as shown by lit-pa7^-lit structure. 



Development of Banded Orthogneisses as a Result of Suc- 

 cessive Injections. 



The foliation of the granite gneisses was once looked 

 upon as an evidence of sedimentary origin. With the 

 recognition of their igneous nature and the evidences of 

 later mashing, a contrary interpretation very generally 

 came in, regarding the foliation as wholly due to rock 

 flowage in the solid state. Observations began to multi- 

 ply, however, which showed that a very appreciable 

 portion of gneisses owed their parallel structure to 

 flowage while in the fluid or partly fluid state — the 

 protoclastic structure of primary gneisses. This is 

 contrasted with what Becke called the crystalloblastic 

 structure — that developed by rock flowage — in the true 

 compression gneisses. Adams especially has shown the 

 importance of deformation during crystallization for the 

 Laurentian gneisses, and Leith in his Structural Geology 

 calls attention to the dominance of protoclastic gneisses. 



It is desired here to emphasize a somewhat different 

 phase of the subject which may be called intermittent 

 injection and crystallization in batholithic roofs. It 

 seems to be associated especially with regions under 

 differential crustal compression, and gives banded ortho- 

 gTieisses.^^ It may grade from the previously described 



-'' [The nomenclature of the gneisses is in danger of confusion. It is well, 

 following Van Hise, to make the term gneiss one of purely structural sig- 

 nificance. Eosenbusch includes in the main division gneisses of igneous 



