THE BASIC MASSIVE ROCKS OF THE LAKE 

 SUPERIOR REGION. 



III. SKETCH OF THE PRESENT STATE OF KNOWLEDGE CONCERN- 

 ING THE BASIC MASSIVE ROCKS OF THE LAKE 

 SUPERIOR REGION. 1 



Without attempting to distinguish critically between the 

 different types of the basic rocks occurring in the Lake Superior 

 region, it will be sufficient for the present to call attention to 

 some of the work done on them, more especially with reference 

 to their microscopical examination. It will not be necessary to 

 refer to all of the articles in which the "traps" of the region 

 have been more or less briefly mentioned, as it will serve our 

 present purpose to allude only to the most important papers on 

 the subject, and to outline, where advisable, the descriptions of 

 the most important rocks as given by various authors. Professor 

 Irving 2 has discussed the theories held by some of the writers 

 with respect to the origin of the traps, but since these, when they 

 differ from the generally accepted theory of an igneous origin 

 for the rocks in question, are found to be opposed to the facts 

 observed, it would be unprofitable to discuss them further. There 

 can be no doubt but that all of the basic, massive rocks found in 

 dykes and beds in the Lake Superior region are truly igneous. 



Douglass Houghton 3 first called attention to the wide-spread 

 occurrence of traps around Lake Superior in his Fourth Annual 

 Report as Geologist of Michigan. He identified knobs, dykes 

 and flows of trap, but was unable to distinguish between the 

 numerous varieties of the rock. His observations related prin- 

 cipally -to the traps in the Archaean and Keweenawan areas in 

 Michigan. 



I This Journal, Vol. I., p. 433. 



2 The Copper-Bearing Rocks of Lake Superior. Monographs U. S. Geological 

 Survey, Vol. V., p. 7. 



3 Dated 1841. Reprint in Memoir of Douglass Houghton, by Alvah Bradish, 

 Detroit, 1889, pp. 167-168, and 176-182. 



587 



