41 6 N. L. BOW EN 



the earlier crystals of plagioclase are basic bytownite, they are, in 

 nearly all cases, gradually made over into labradorite by the liquid 

 in which they remain suspended." The statements could hardly 

 be more specific, and Grout's reference (with its accompanying 

 objections) to my statement regarding a simple mixture of anorthite 

 and albite is hardly to the point when such definite consideration 

 of the actual case of gabbroid magma has been offered. We have 

 seen then that the specific objections to crystal settling that Grout 

 offers are in part due to a misconception of that process, and that 

 in many cases where he supposes a convective action to have 

 advantages over settling the advantage has no real existence as 

 far as explanation of the phenomena of the Duluth gabbro is 

 concerned. However, it must be conceded that Grout's main 

 contention is substantially correct, viz., 'that the theory of crystal 

 settling in the simple and generalized form in which it has been 

 stated does not offer an adequate explanation. We have seen that 

 his convective action is likewise inadequate. As an offset to so 

 much destructive criticism a constructive suggestion will be 

 ventured. The suggested explanation will involve two principal 

 assumptions: first, that the igneous mass of the Duluth lopolith 

 was injected as a sensibly homogeneous and completely liquid 

 magma, and secondly, that the action which produced the basin- 

 like form of the mass took place in part contemporaneously with 

 the period of crystallization of the magma. The first of these 

 conclusions is, it should be remarked, in complete accord with 

 Grout's conclusions. Under five heads Grout points out the impos- 

 sibility of belief in the intrusion of heterogeneous liquid, a process 

 that the writer has suggested as a possible result of the squeezing 

 of liquid from a crystallizing mass.^ This suggestion should be 

 applied only to cases where the evidence is strong that there were 

 two liquids. I am wholly in accord with Grout that it is a quite 

 inadequate explanation of the banding of the Duluth mass and 

 indeed of most rocks showing primary banding. Usually such 

 banding is to be referred to movement during crystallization.^ 



'^ N. L. Bowen, "The Later Stages of the Evolution of the Igneous Rocks," Jour. 

 GeoL, XXIII (1915), Supplement, pp. 27 and 83. 



^N. L. Bowen, "The Problem of the Anorthosites, " Jour. GeoL, XXV (1917), 

 236 and 237. 



