ACADEMY OF 8C.EKO.] AMERICA. 55 



nepheline-bearing and analcitic alkaine rocks. At Sudbury the pre-Animikiean micronorites ' 

 show pillow-structure, and are associated with greenstones and altered tuffs. In Keweenawan 

 times was formed the Sudbury complex, which is similar to the Duluth complex though smaller, 

 and has probably had an essentially similar origin. Coleman ('05, '07) followed by Daly ('14) 

 considers it to be a clear instance of a gravitationally differentiated laccolite bent into a spoon- 

 shaped syncline. A remnant of the sediment under which it was injected remains in the center 

 of the trough. The lower portion of the mass is a dark-gray, coarse-grained norite grading up 

 into pale gray or flesh-colored quartz-norite, followed by grano-diorite and granite which forms 

 the upper portion, the .-total thickness being about 6,500 feet. Barlow ('04), however, believed 

 that the granite is intrusive into the norite and Dr. Harker ('16) remarks on the occurrence of 

 hybrid rock types between the norite and granite, which otherwise appear rather sharply 

 separated. The basal portion of the norite contain nickeliferous sulphides, veinlets of which 

 project into the underlying schists, etc. These sulphides have been thought to be basal gravi- 

 tational differentiates from a magma with which their miscibility was limited, but are now 

 thought to be later perhaps hydrothermal concentrations (Tolman and Rogers '16). An elabo- 

 rate discussion of their origin from a metallographic point of view has been given by Goodchild 

 C18). 2 



In Eastern Canada the Archaean rocks have been divided into two series, the Grenville 

 and the Laurentian gneisses. The Grenville series consists of limestones, quarzites, etc., and 

 amphibolites and some gabbros. The amphibolites are in part the hornblendic alteration prod- 

 ucts of tuffs and basic flows derived from centers of eruption near the occurrences of the gabbros, 

 which are sometimes considerably differentiated into felspathic, ilmenitic, and pyroxenic 

 phases. In opposition to Daly's ('14, p. 327) view, Adams and Barlow ('06, p. 154) hold there 

 is no regular arrangement of rock-types observable. The whole area is greatly folded and 

 invaded by domed batholiths of gneissic granite, fringed with nepheline-syenites along the lines 

 of contact with the limestone. (See Dr. Harker's ('17) explanation of this.) 



In the Hastings district and still further to the east masses of anorthosite and other basic 

 igneous rocks appear among the basic lavas, which, like the above, may belong to the Keewatin 

 series, and these masses are believed by Daly ('14) to be essentially laccolitic in character. 

 The Chibougamou mass, for example, which is extensively differentiated, consisting of gabbro, 

 norite, pyroxenite, and iron ores, makes concordant contacts with the invaded greenstone- 

 schists (Barlow '11). Other masses are found in eastern Canada in the Saguenay and Morin 

 areas, and in the latter they are associated with probably consanguinous but rather later intru- 

 sions of syenite. These occurrences Bowen ('17) believes to be analogous with those of anortho- 

 sites in the Adirondacks. He pictures the last as the middle member of a differentiated laccolite. 

 The lower concealed portion consists presumably of gabbro, the upper of syenite which, though 

 produced by gravitational differentiation in situ, has been occasionally thrust intrusively into the 

 underlying anorthosite. The margin of the laccolite shows a cooling-selvage of gabbro. The 

 assumed mechanics of differentiation are however peculiar. After the consolidation of the cooled 

 gabbroid selvage "the femic crystals separated by gravity from the gabbro-magma, and the 

 plagioclase, then bytownite, remained practically in suspension in the melt. When the liquid 

 had become distinctly lighter, having attained a diorite-syenite composition, the plagioclase 

 crystals, now labradorite, accumulated by sinking and give masses of anorthosite, at the same 

 time leaving the liquid out of which they settle of a syenitic or granitic composition." Cushing 

 ('17), while accepting Bowen's general conclusions as to the form and origin of the intrusion, 

 holds that the syenite is distinctly subsequent to the anorthosite, though consanguinous, and 

 that it consists of a number of separate intrusions, and did not form a single sheet beneath a 

 roof of chilled gabbro, which separated from the parent magma of the anorthosite, supposed by 

 Bowen to form a continuous sheet-laccolite, underlying the whole Adirondack region. Bowen 

 in a rejoinder ('17) suggests that these apparently isolated masses of syenite may occupy marginal 

 irregularities in the laccoli tic-roof. Miller ('18) inclines more to the view of Cushing. He 



1 This important paper came to the writer's notice too late to be considered in the preparation of the present article. 



