IOWA ACADEMY OF SCIENCES. 145 



problems of correlation, it remained for Irving* to point out its 

 great value in the classification of the nonfossiliferous, pre- 

 Cambrian rocks of the Lake Superior region. The practical 

 use of this criterion in stratigraphy was also later invoked in 

 the consideration of the Carboniferous of the Mississippi 

 valley f 



The special stress laid by Irving on the value of unconformi- 

 ties as a basis for geological classification, has a wide bearing. 

 In the application of the principle to the region that was 

 Tinder consideration, it was shown that unconformities were 

 the most important of all criteria in resolving into its grander 

 subdivisions, a vast sequence of crystalline rocks, which, as in 

 the case of other similar masses, had defied all attempts of sat- 

 isfactory arrangement and correlation. Had Irving not been 

 so untimely called from his field of labor, he might have possi- 

 bly expanded his theme so that it would be of much wider, if 

 not of universal application. It is not that he was the first to 

 suggest the use of unconformities in delimiting the grander 

 geological formations, for this, at the present time, is essenti- 

 ally the real foundation of our accepted geological classifica- 

 tion. Other criteria, however, have so overshadowed this one, 

 that the fact of its ever having assumed an important role is 

 well-nigh lost sight of, and consequently the physical 

 breaks in stratigraphical succession excite little attention, 

 except as interpreted by fossils. Their true significance is 

 now very nearly, if not completely, overlooked. 



In the absence of fossils, Irving was actually driven to the 

 use of purely physical methods in dealing with the metamor- 

 phosed rocks. All attempts to arrange the latter systemat- 

 ically, except upon faunal grounds, had been given up as use- 

 less. In other regions, many writers before him had con- 

 sidered the phenomenon of marked discordant sedimentation 

 as a structural feature, and had actually gone so far as to 

 regard unconformities as not only of regional, but even of 

 intercontinental, extent. On the other hand, there were a 

 very large number who believed that unconformities, at best, 

 were only local phenomena and, therefore, of small importance 

 in stratigraphy. It was Irving's particular mission to 

 determine how far unconformities could be relied upon, in a 

 limited district, to point out clearly that in some cases they 



*U. S. Geol. Sur., Seventh Ann. Rep., pp. 437-439. 1888. 



+BuU. Geol. Soc. American, Vol. Ill, pp. 283-300, 1892; and Iowa Geol. Sur., Vol. II, p. 



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