REVIEW OF NOMENCLATURE OF KEWEENAWAN 

 IGNEOUS ROCKS^ 



ALEXANDER N. WINCHELL 

 University of Wisconsin 



The Keweenawan igneous rocks of the Lake Superior region 

 have been studied and discussed by many geologists during the past 

 thirty years. At the beginning of that period microscopical petrog- 

 raphy was in its infancy, and minor errors due to faulty methods 

 inevitably resulted. In the course of the years these have been 

 gradually corrected, involving changes of nomenclature. Some 

 variations in nomenclature have resulted from the varying points 

 of views of the authors. But the general progress of petrography 

 has brought more numerous and important modifications. 



In order to make the names used by the prominent wTiters on 

 the subject more readily intelligible, a correlation of these is pre- 

 sented herewith. It must be remembered that, since the basis of 

 petrographic classification used by the authors has varied somewhat, 

 such a correlation can be only an approximation, but it will never- 

 theless serve the purpose of showing the various changes that have 

 occurred, and of presenting, at least in its outlines, the main facts of 

 nomenclature of each writer. 



In order to give precision to such a correlation, it is desirable that 

 the nomenclature of each writer be compared, not simply with that 

 used by other authors, but also with an expressed and definite classi- 

 fication. Therefore the following classification has been prepared, 

 on the basis of textures and mineral composition. It is not a general 

 classification of igneous rocks, but is intended to include merely the 

 types represented in the Keweenawan of the Lake Superior region. 



Macfarlane,^ in 1866, described the Keweenawan rocks of Michi- 

 picoten Island. He found melaphyre, trap, amygdaloid, quartz por- 

 phyry, porphyrite, and trachytic phonolite. His " quartz porphyry," 



I Published by permission of the Director of the U. S. Geological Survey. 

 3 Thos. Macfarlane, Geol. Siirv. of Canada, 1863-66, 1866. 



765 



