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IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
rigidly to test the applications of the explanatory theories and 
to make impartial comparisons between deduced consequences 
and generalized records. To a degree of stupidity almost, it 
seemed at times, they had to be shown in the field the detailed 
proofs. They not infrequently gave scant consideration to 
many trivial features of which far too much had been manifestly 
made. They brought to bear upon intricate problems the in- 
valuable experiences of other lands. They were better able to 
view things more broadly than is possible in the cases of those 
who had worked mainly in limited areas. Altogether what they 
agreed upon was often quite different from what had been pre- 
sented to them, piecemeal as it were, in isolated localities. 
In the various attempts to resolve the most ancient sedimen- 
tary successions into their terranal elements the one great 
drawback is, of course, the more or less highly metamorphosed 
state of the rocks. This difficulty is even more serious than 
the one which confronted the student of the Paleozoics before 
use was made of fossils. Among very old formations it is often 
hard to distinguish between rocks which have been altered from 
igneous masses and those which have been changed from sedi- 
mentaries. Moreover, without some scheme of taxonomic loca- 
tion of rock-masses through the consideration of which critical 
criteria of stratigraphic recognition are proposed, developed 
and modified, very erroneous notions of mass relationship must 
prevail. In many places, notably, for instance, on the Atlantic 
Piedmont plateau, much of the supposed Pre-Cambrian com- 
plex recently proves to be merely highly altered Cambric, Ordo- 
vicic and even Siluric sediments. On the other hand, thick 
sections of strata in the Selkirks, for example, long regarded 
as Paleozoic in age are found to have beyond all doubt, Pre- 
Cambrian affinities. 
Ever since it has come to be appreciated that there is really 
a Pre-Cambrian stratigraphy the great desideratum has been 
the discovery of some spot on earth where the ancient beds still 
remain but little altered and where a definite terranal sequence 
may be made out in the same way that it is in the case of the 
newer sedimentary successions. Several such locations are now 
known; one on the north shore of Lake Superior, and another 
in the Rocky Mountains on the boundary between the United 
States and Canada, are particularly noteworthy. In neither 
