594 THE JOURNAL OF GEOLOGY. 



for a knowledge of the field relations of the specimens studied. 

 Messrs. Herrick, Clarke and Deming 1 have also studied a few 

 specimens of the gabbro, both ordinary and orthoclastic varieties, 

 from Duluth, but they have added little to what was already known 

 concerning them, except the suggestion of the possible depend- 

 ence of the orthoclase-bearing varieties upon their environment 

 for the peculiar characteristics which they possess. 



The Canadian geologists have likewise been engaged in a 

 study of the rocks on the north side of Lake Superior. Many 

 allusions have been made to the massive sheets and dykes in the 

 Thunder Bay region, but no microscopical descriptions of them 

 have been published, with the exception of a few notes by the 

 present writer appended to a report by Mr. Ingall 2 on Mines 

 and Mining in the Thunder Bay Silver District. In this report 

 the relations of the large dykes and thick beds of diabase or 

 gabbro to the fragmental rocks of the Animikie series north of 

 the lake are carefully sketched, and the microscopic features of 

 the most important rocks are described. In the Appendix, 3 a 

 few altered gabbros and diabases from both sheets and dykes are 

 very briefly characterized. The former of these have the general 

 peculiarities of the gabbro from the great dyke on Pigeon 

 Point, Minnesota, referred to by the writer 4 in an article on 

 certain contact phenomena at this place, and described at greater 

 length 5 in a bulletin of the U. S. Geological Survey. In the 

 first of these two papers, in addition to the reference to the 

 Pigeon Point dyke, a few remarks are made concerning the rela- 

 tions of Irving's orthoclase-gabbros to the more common varie- 



1 C. L. Herrick, E. S. Clarke and J. L. Deming : Some American Noiytes and 

 Gabbros. Am. Geol., June, 1888, p. 339. 



2 E. D. Ingall : Report on Mines and Mining on Lake Superior. Geol. and 

 Nat. Hist. Survey of Cannda. Montreal, 1888. 



3 W. S. Bayley : Notes of Microscopical Examination of Rocks from the 

 Thunder Bay Silver District. 



4 W. S. Bayley : A Quartz-Keratophyre from Pigeon Point and Irving's Augite- 

 Syenites. Am. Jour. Sci. XXXVII., 1889, p. 54. 



5 W. S. Bayley : The Igneous and other Rocks on Pigeon Point, Minnesota, 

 and their Contact Phenomena. Bull. No. 109, U. S. Geol. Survey, 1893. 



