THE BASIC MASSIVE ROCKS, ETC. 595 



ties of the gabbro of the region, but no detailed descriptions of 

 these rocks, nor of the ordinary gabbros, whose modified forms 

 they are supposed to be, are given. Finally, Dr. A. C. Lawson 1 

 has mentioned some of the characteristics of certain diabases 

 from dykes in the Archaean rocks of the Rainy Lake region, in 

 which the gabbroitic as well as the diabasic structures are well 

 exhibited, the former toward the centers and the latter near the 

 sides of the masses. 



The most comprehensive treatment of the " greenstones " 

 and "greenstone schists" of the Lake Superior region is that 

 by Dr. G. H. Williams 2 in his bulletin on the origin of the 

 green schist, supposed to underlie the Huronian in Michigan. In 

 this volume the author not only describes the petrographical 

 features of the schists with which he deals, but he likewise 

 describes in some detail the microscopical characteristics of the 

 diabases, diabase porphyrites, ^diorites, diorite porphyrites and 

 gabbros, associated with the schists, and from some of which the 

 latter have been derived. 



Within the past three years a number of papers have 

 appeared in which reference is made to some of the special 

 features of a few of the coarse basic rocks, both north and 

 south of the lake, but no articles have been published that deal 

 with their general features. Fairbanks 3 has communicated a 

 few notes on the diorites and gabbros in the province east of the 

 north side of Lake Superior. Irving and Van Hise 4 have given 

 a brief synopsis of the characteristics of the diabase dykes and 

 interbedded sheets in the Penokee iron series on the south side 



1 A. C. Lawson : Notes on Some Diabase Dykes of the Rainy Lake Region. 

 Proc. Can. Inst, for 1887, and Report on the Geology of the Rainy Lake Region. Pt. 

 F., Ann. Rep. Geol. and Nat. Hist. Survey of Can. for 1887-88, pp. 57-73 and 147-164. 



2 G. H. Williams : The Greenstone Schist Areas of the Menominee and Mar- 

 quette Region of Michigan. Bull. No. 62. U. S. Geol. Survey, 1890. 



sfL W. Fairbanks : Notes on the Character of the Eruptive Rocks of the Lake 

 Huron Region. Amer Geologist, I. 1890, p. 162. 



4 R. D. Irving and C. R. Van Hise : The Penokee Iron-bearing Series of 

 Northern Wisconsin and Michigan. Monograph XIX., U. S. Geol. Survey, 1893. 

 Chap. VII., The Eruptives. 



