262 FRANK F. GROUT 



was deformed, the crystals would be broken and bent in rather 

 violent fashion. Rock fiowage would be evident in the examination 

 of the grains. It is possible, of course, that the temperature is so 

 high and conditions so favorable for recrystallization that some 

 of the strain effects would later disappear, but the broken crystals 

 of porphyries and such rocks as the Sudbury norite argue against 

 such effects. Even if recrystallization occurred, the process would 

 be accomplished under stress and an orientation of grain would 

 be almost inevitable. It is worth while, then, to investigate the 

 cases of supposed filter-pressing for signs of structure and deforma- 

 tion. It would be a strong confirmation of Bowen's argument if a 

 laccolith or sill could be cited in which the fluxion structure corre- 

 sponded to the position of such differentiates as he describes. 

 Neither Bowen, Judd, Barrow, nor Harker makes any mention of 

 such an orientation. On the contrary, in the illustrations of the 

 glass supposed to have oozed into vesicles Judd and Harker show 

 exactly the opposite orientation. It looks much more as if a lump of 

 solid glass had interfered with the haphazard position of the crystals. 

 Turning to the special case of Duluth, Bowen takes up a final 

 suggestion of the mechanics of filter-press action by proposing a 

 theory to account for the rocks described.^ The great lopolith 

 shows a sunken structure at present, and on that basis it may be 

 assumed that there were some movements during crystallization.^ 

 Bowen beUeves that movements when the mass is 50-65 per cent 

 crystalHne would tend to produce bands and layers by the bridging 

 action of the feebly interlocked crystal mesh. The reality of the 

 bridging effect in a crystal mesh need not be questioned, but a 

 more detailed report of the experimental work which led him to the 

 suggestion and to the estimate of 50-65 per cent of crystals would 

 be welcome. He has previously^ stated an opinion that a magma 

 with 50 per cent crystals was eruptible, and the bridging effect 

 was ignored. 



'7rar. Geo/., XXVII, p. 411- 



= The main subsidence must have occurred before crystallization, for the mass is 

 too thick to be supported isostatically as a dome. It is not improbable, however, that 

 movements continued. 



^ Jour. Geo!., Supplement, November-December, 1915, p. 31. 



