Reviews — Crystallization-Differentiation. 87 



Again, Dr. Bowen considers that abrupt transition would not be 

 expected on the immiscibility theory, since an emulsion would result. 

 Where immiscibility has been claimed inMull as yielding differentiates 

 in place perfect transitions occur (Summary of Progress for 1913, 

 pp. 48, 49, 51). Of course, transition may be expected on any 

 theory of gravitational differentiation. 



Using the Duluth lopolith as an example, Dr. Bowen discusses the 

 gabbro-granophyre association and claims that this discontinuous 

 variation can be explained by the mechanical straining off of the 

 mother liquor (granophyre) from the already solidified crystals 

 (gabbro) under the influence of pressure. This mechanism is, of 

 course, well known in Britain, though its advocacy by Mr. George 

 Barrow nearly thirty years ago in relation to certain intrusions in 

 the East Highlands (Geol. Mag., 1892, p. 64, and Quart. Journ. 

 Geol. Soc, 1893, p. 330). 



Dr. Bowen's suggestion (p. 417) that banding of gabbros and 

 peridotites is not a form of flow-structure will scarcely receive 

 credence. Here the likeness of Dr. Bowen to M d'Astarac becomes 

 especially close. He believes that banding is a reaction to 

 mechanical stress on a crystal mesh with still fluid matrix, and that 

 the stress tends to give rise to cavities into which the' matrix exudes, 

 while the crystal mesh above and below packs. One can scarcely 

 doubt that such an origin would show itself in an orientation of the 

 early minerals with their axes perpendicular to the exudation bands 

 which they border. One has only to look on a large surface of a 

 banded rock — especially of a porphyritic banded rock — to perceive 

 that the mechanism evoked by Bowen has not operated, but that 

 true flow has occurred. Moreover, Dr. Bowen's explanation, on his 

 own theory, would apply only to an ultrabasic rock banded with 

 more acid rock and not to the very common case in which gabbros, 

 for instance, are banded with scarcer ultrabasic layers. 



Dr. Bowen discusses the origin of nodules and schlieren in igneous 

 rocks, and considers them to be due to crystal jams resultant from 

 the packing together of crystals at constricted points and their 

 subsequent removal and partial resorption in the main body of 

 the magma. Such filter presses, required here and elsewhere by 

 Dr. Bowen, have been found to be quite unworkable when copied 

 in industrial processes. 



This paper must be welcomed chiefly because it will rouse claimants 

 of liquid immiscibility to renewed exertions. There is little, if any, 

 tendency on this side of the water to rule out either settling of crystals 

 (postulated seventy years ago by Darwin) or mechanical straining 

 (postulated thirty years ago by Barrow) as factors in differentiation, 

 but there is a tendency, perhaps a growing one, amongst field- 

 workers to support the claim that liquid immiscibility is a vera causa 

 in addition. Field observations will supply, one beUeves, the key to 

 differentiation. 



H. H. Read. 



