312 THE JOURNAL OF GEOLOGY. 



reinforced in the latter case by his map of Northwestern Minnesota, which 

 suggests that the upper division of the Keweenawan overlaps the lower uncon- 

 conformably. 



In the first of these areas the thin -bedded flows are described as being 

 poured out against the gabbro mass, which, it is said, must have stood to a 

 great height, until finally the flows accumulated sufficiently to cap the upper 

 surface of the gabbro. So strongly was Irving impressed by these facts, that 

 he states that he was inclined at first to place the gabbros of the Bad River 

 district with the Huronian, and to regard them as the equivalents of the 

 great flows of the Animikie series of Thunder Bay, but, finding the Animikie 

 slates unconformably under the gabbros, he preferred to put them as the ear- 

 liest division of the Keweenawan, clearly recognizing that there was a very 

 considerable unconformity between these coarse, massive rocks and the later 

 thinly -bedded ones. This reference was made because of the close lithologi- 

 cal relationship of the gabbro and the Keweenawan diabases, and because in 

 eruptive series such breaks were regarded as less significant. 



It thus appears that Irving fully appreciated an unconformity, probably at 

 the horizon of Lawson's unconformity, but he did not recognize that the break 

 which has so extensive an occurrence also exists along the Minnesota coast. 

 If the explanation suggested as to the relations on the Minnesota coast be true, 

 Irving's statements, used in reference to the Bad River area, can be applied 

 almost exactly to this one, in which case the difference between Irving and 

 Lawson is that of nomenclature. Lawson restricts the term Keweenawan to 

 the upper part of the series, whereas Irving and other writers regarded the 

 Keweenawan as including both divisions. It also follows, if the anorthosite is 

 Keweenawan, that Lawson's conclusion that the Animikie is absent below the 

 Keweenawan in Northeastern Minnesota, is without foundation, for the base 

 of the Keweenawan thus defined is not here exposed. Further, the Animikie 

 is certainly unconformably below the great basal gabbro of Minnesota. It 

 further follows that the correlation of the anorthosites of Lake Superior and 

 those of the Province of Quebec has no value. But, wholly apart from the 

 stratigraphy of Northeastern Minnesota, I must confess to a complete lack of 

 confidence in the correlation of eruptive rocks so far removed as these. 



To the statement that Irving's subdivision of the Keweenawan into groups 

 and his estimate of the thicknesses of various portions of the series are of 

 little value, I feel that I must take exception. The painstaking character of 

 all of Irving's work is well - known. He spent many years of study upon 

 the series in Michigan and Wisconsin. His study, and that of his assistants, 

 Messrs. Chauvenet, Cambell and McKinley, on the northwest coast of Lake 

 Superior was of a detailed character. It would seem scarcely possible that 

 Dr. Lawson's study of the stratigraphy in a single trip, in which he made no 

 attempt to re - measure the sections (so far, at least, as can be ascertained from 

 his paper) could have been detailed enough to warrant this sweeping state- 



