MOVEMENTS IN CRYSTALLIZING MAGMA 259 



if -brought down by convection. In this he is wholly correct and 

 no advantage is claimed for convection. It is only when the band- 

 ing is accompanied by an orientation of grain that the evidence 

 is strong against crystal settling. It is not the banding nor the 

 parallelism which determines the process, but the combination of 

 bands with a fohated structure. This was emphasized in the 

 original papers.' As there stated, crystal settling is to be thought 

 of as a matter of a few feet, not thousands of feet. 



FILTER-PRESSING 



A process of expulsion of residual fluid magma from a mass after 

 a large proportion of the material had crystallized is advocated by 

 Bowen and Harker. Preliminary suggestions of the idea are cr edited 

 by Harker to Barrow and to Judd (who refer also to Osann, Teall, 

 and Geikie). 



In tracing these earlier statements of the idea, no good outline 

 of the mechanics or general results could be found. Teall and 

 Osann and Harker^ describe patches of glassy or residual magma 

 filling steam cavities when the gas of the cavity has been absorbed or 

 condensed, or has escaped. No one can question the occurrence of 

 rounded lumps formed from magma, but the source of the lumps 

 and the process by which they got there, and the ultimate removal 

 of any gas that may have been there, are hard to understand. If 

 the magma was Hquid enough to "ooze out from among the crystals" 

 the gas just separated from the magma could hardly be absorbed, 

 condensed, or allowed to escape, leaving a spherical hole. Barrow^ 

 has described a pegmatitic separation from granite, as have many 

 others. In many cases this may be a process of separation of 

 Hquid from partly crystalline magma, but pegmatites are most 

 readily explained by the readier penetration of thinly fluid emana- 

 tions in advance of viscous magma. This is more nearly analogous 

 to immiscible separation than to filter-pressing. Filter-pressing 



'Frank F. Grout, "Internal Structures of Igneous Rocks," Jour. GeoL, XXVI 

 (i9i8),p. 455. 



' A. Harker, The Natural History of Igneous Rocks, pp. 324-25. Macmillan, 1909. 



3 George Barrow, "On Certain Gneisses and Their Relation to Pegmatites," 

 Geol. Mag., 1892, pp. 64-65. 



