692 THE JOURNAL OF GEOLOGY. 



It was not until a few years since that an attempt was made 

 to discover the true relations of these gabbros to the surrounding 

 rocks. In his Copper-Bearing Rocks (p. 266) Prof. Irving 

 places them at the base of the Keweenawan group, at the same 

 time stating that "There is no definite evidence of unconformity 

 between the gabbros and the slates of the Saint Louis River," 

 regarded as Animikie. In a later paper the same writer 1 refers 

 to a coarse-grained, stratiform olivine-gabbro at the base of the 

 Keweenawan. 



Though nowhere so stated, the olivine-gabbros had by this 

 time been separated by the author from the overlying "ortho- 

 clase gabbros," and had been placed by him at the very base of 

 the Keweenawan group, with the orthoclase-gabbros immediately 

 above them. In his article 2 on the classification of the early 

 Cambrian and pre-Cambrian formations, we have this description 

 of the position and nature of this great mass of rocks, ". 

 We find at the base of the series [Keweenawan] an immense 

 development of stratiform, fresh and often exceedingly coarse 

 olivine-gabbro, the individual layers of which, notwithstanding 

 their complete crystallization, very coarse grain, and lack of 

 amygdaloidal or dense upper surfaces, seem evidently to have 

 formed great flows at the surface of the region as it stood at the 

 time of their extrusion." 



No more explicit statements of his views concerning this 

 basal gabbro appear in any of Irving's writings. A reference to 

 the geological map of north-eastern Minnesota accompanying the 

 paper last referred to, will, however, show that at this time ( 1886) 

 he believed the basal gabbro in Minnesota to rest unconformably 

 upon the Animikie, since the former is represented as cutting 

 transversely belts of St. Louis slates, the Mesabi granite and 

 schists of the Archean, and the eastern area of Animikie slates 

 along the boundary line between Minnesota and Canada, which 

 slates here strike nearly east and west. 



Although in his maps the "gabbro flow" is represented as 

 'Am. Jour. Sci., 3d ser., vol. 34, 1887, pp. 204, 249. 

 2 Seventh Ann. Rept. U. S. Geol. Survey, 1888, p. 419. 



