AMERICA. 



53 



spread at or near the base of the flows, and along the unconformity at the base of the Kewee- 

 nawan sediments a little below the flows." Generally the mass is regarded as a laccolite, but 

 a recent study by Grout ('IS) has led to the conception of a somewhat different form of intrusive 

 mass which he terms a ''lopolith." The nature of this is illustrated in figure 16. The detailed 

 penological study of this mass has been summarized by Winchell ('11) and Grout ('ISc). We 

 are here concerned with the disposition of the various types of rock within the mass. The 

 following comments are based chiefly upon Grout's recent papers ('18, 'ISa, '18b, '18c). 

 It is estimated that over two-thirds of the gabbro mass at Duluth consists of olivine-gabbro, 

 varying only slightly from the average. Such average rocks are scattered from top to bottom, 

 the gabbro-mass being over 3 miles thick. The intrusion of the gabbro occurred in two or 

 more events, for chilled contacts and apophyses of the newer gabbro in the subordinate and 

 more feldspathic older gabbro are clear in some regions. Specialized rock-types have a more 

 limited range. The peridotite occurs only near the base; the equally heavy magnetite- 

 gabbro is near the center. The anorthosite ranges from the center toward the top, and is 

 in the thin earlier intrusion. Very locally at the base of the early gabbro there is an 

 apatitic hypersthene-gabbro. A red granophyre occurs mostly near the top and in a sill at a 

 higher horizon. Grout points to the surprising uniformity of the composition of the feldspar 



XXXXXXXXX 



X 

 X 



X X N— 



XXX*, 



Fiq. 16. — Relation of a lopolith to invaded sediments. (After Grout.) 



(bytownite) throughout the complex and argues that this indicates a continuous adjustment 

 of the composition of the (unzoned) feldspar, and, in consequence, the absence of any settling 

 out of plagioclase more basic than the average feldspar content of the magma. Only in the 

 upper portions are zoned or more acid feldspars known. Nor does the specific gravity of the 

 rock diminish regularly toward the higher portion of the mass. Indeed one of the heaviest 

 rocks described is well above the center. Weinschenk ('16) is of the opinion that this is the 

 general rule for the segregation of magnetite, part of which remains molten till a late stage in 

 magmatic consolidation (Grout '18c). (Compare Duparc and Pearce '05 and, for a volcanic 

 instance, Jensen '16.) Nor is there any long series gabbro-diorite-syenite-granite ; nor gabbro- 

 quartz-diorite-granite, as Bowen's hypotheses seem to suggest. On the contrary, rocks of 

 intermediate composition are markedly absent (as also in the case of the Bushveld complex) 

 and the frequency of this feature has been emphasized by Dr. Harker ('16). The separation 

 of the basic and acid types is so sharp as to suggest to Grout that the upper portion of a grano- 

 phyric composition separated as an immiscible liquid from the underlying gabbro. In the 

 field this change comes with surprising abruptness after the monotony of slightly varying 

 gabbro-bands. In a few feet after the reddish tinge of granophyre is seen in the interstices 

 of the gabbro, none of the gabbro-minerals are visible in the rock. The chief outcrops of the 

 granophyre are irregular patches at the top of the main gabbro and apophyses into its roof; 



