54 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 1MEMO,RS t voL T .xlx, 



it occurs also near the top of the earlier feldspathic gabbro, in a large sill close above the gabbro 

 and in some small disks near the bottom of the gabbro. In a sill near Duluth there is a remark- 

 able example of perfect gradation from diabase to granophyre. The sill is 1,500 feet thick, but 

 the zone of gradation from black diabase to intensely red granophyre is less than 50 feet thick. 

 The granophyre is sugary with miarolite cavities. Grout believes that the granophyre contains 

 too much alkali for it to be considered a syntectic of gabbro and acid sediments * and holds that 

 it was concentrated at the top of the magma chamber by the action of water and other vapors, 

 the magma so produced becoming immiscible with the underlying less hydrous gabbro-magma. 

 This latter is strongly banded in zones parallel to the margin of the intrusion, which structure 

 Grout considers to have been produced by the action of convection-currents in the crystalliz- 

 ing magma. 



Bowen's ('19) alternative explanation of this complex is of interest. The experimental 

 evidence against immiscibility of liquid magmas is strongly urged. It is held that the Duluth 

 laccolite may have originated from " a gabbroid magma, wholly liquid, injected into its present 

 place." Gravitational separation of olivine and pyroxene gradually accumulated on the floor 

 of the laccolite a thick layer of crystals, which at the base were packed together almost to 

 the exclusion of molten matter, but higher up a crystal-mesh contained interstitial magma. 

 The bulk of the iron-ores did not separate from the magma till crystallization was well advanced, 

 and therefore sank to rest not on the floor of the magma, but above much of the layer of olivine 

 and pyroxene. Intermittent deformation developed a series of lenticular openings which in- 

 stantly filled with liquid from the interstices of the weakly-knit crystal-mesh. " The crystals 

 that became detached during the shearing would naturally become aligned in the liquid filling 

 the lenses, so that fluxion-structure would be a natural accompaniment. Moreover, in the 

 larger lenses developed, a gravitative sorting of these detached crystals would take place under 

 particularly favorable conditions, and bands showing the extreme contrast of monomineralic 

 types might thus be formed." (Bowen '20.) Thus banding as a primary feature was produced 

 as this process was repeated many times by continuous warping. The conditions favorable to 

 its accomplishment are present when the magma contains from 50 to 65 per cent crystals. When 

 the upper limi t had been passed and the whole mass approached complete crj'stallization, it 

 was able to withstand compression until the accumulated lateral force exceeded its power of 

 resistance and produced an upward squeezing of the residual liquid, thus interrupting the course 

 of regular differentiation outlined in the earlier paper (Bowen '15) and accounting for the sharp 

 separation of basic from acid rock. The differentiation would be markedly discontinuous 

 the later differentiate would have an intrusive relation into the earlier in some places, and a 

 rather abrupt transition in others, and yet the relations would not indicate successive intrusion 

 in the ordinary sense of the term. In so far as this action is incapable of occurring before a 

 certain degree of crystallinity has been attained, there should be a tendency to fairly constant 

 contrast between the two differentiates. On the other hand there is no necessity for the con- 

 stant relative proportions in their amount. "The gabbro-granophyre association fulfils every 

 requirement." The uniform composition of the plagioclase is not, according to Bowen, inex- 

 plicable in terms of his hypothesis. 



The Duluth gabbro is similar in many ways to the Bushveld complex, in which, however, 

 the acid differentiate is much more abundant, so far as the areal extent may indicate, and we 

 have therefore discussed it at some length; the further comment on Bowen's hypothesis as 

 applied to the Bushveld complex will be made later. 



South of Lake Superior, in the Penokee area, the Bad River laccolite is possibly a con- 

 tinuation of the Duluth mass. South again of it is a group of intrusions ranging from granite 

 to peridotite, invading the not very folded Animikiean sediments (Clements '99). 



East of Lake Superior, rocks are found with a similar nature to those at Duluth. The 

 Keewatin lavas are sometimes ellipsoidal and are invaded by Laurentian gneisses including 



i The composition of the feldspar in the granophyre is exactly the same composition as that of the uucrystallized interstitial residuum at 

 this stage of crystallization of a gabbroid magma (Bowen '19). 



