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 
under 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 v/ider, 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. 
+Bull. Geol. Soc. American, Vol. Ill, pp. 283-300, 1892; and Iowa Geol. Sur., Vol. II, p 
29,1894. . .P 
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