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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 "orthoclase 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

  1. ↑ Am. Jour. Sci., 3d ser., vol. 34, 1887, pp. 204, 249.
  2. ↑ Seventh Ann. Rept. U. S. Geol. Survey, 1888, p. 419.
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