THE BASIC MASSIVE ROCKS, ETC. 591 



from Ashland county, Wisconsin; and Wichman 1 published a 

 classification of Huronian rocks based on their microscopical 

 examination. Wichman divided the massive basic rocks into dia- 

 bases, coarse-diabases and diorites. The only other microscop- 

 ical work done in connection with the Wisconsin Survey is that by 

 the late C. E. Wright, published in the second volume of the 

 reports. In this Mr. Wright 2 mentioned the occurrence of a diorite 

 containing augite in the bed of Black river. 



Further, Dr. Wadsworth, 3 in his discussion as to the origin of 

 the jasper and iron-ores of the Marquette region describes briefly 

 the microscopic features of many of the intrusive knobs that are 

 so prominent a feature in the topography of the district. These 

 are declared to consist largely of diabase and coarse basalt, both 

 massive and slightly schistose. 



The investigation of the basic rocks of the region had by this 

 time been sufficiently exact, and the number of specimens exam- 

 ined was large enough to give an idea of the characters of the 

 commonest types occurring there, but these investigations had 

 been undertaken by so many different geologists that no exact 

 correlation between the various varieties discovered was possible. 

 No classification of these could be accomplished until some had 

 examined specimens from all the different localities and had com- 

 pared them with one another. This work was undertaken by 

 Professor Irving 4 in 1881, and was ably accomplished by him in the 

 course of two years. All publications referring to the lithology 

 of the Keweenawan and Huronian formations on both sides of 

 the lake were carefully reviewed, most of the specimens described 

 in them were examined, and the results of this study and exam- 

 ination, together with a great deal of new information gathered 



1 A. Wichman : Microscopical Observations of the Iron-bearing (Huronian) Rocks 

 from the Region South of Lake Superior. lb. p. 600. 



2 CharlesE. Wright : Geol. of Wisconsin, II., 1878, p. 637. 



3 M. E^ Wadsworth : Notes on the Geology of the Iron and Copper Districts of 

 Lake Superior. Bull. Mus. Comp. Zoology, 1881, Vol. VII., p. 36-49. 



4 R. D.Irving: The Copper-bearing Rocks of Lake Superior. Monograph V., 

 U. S. Geol. Survey, Washington, 1883. 



