IOWA ACADEMY OF SCIENCES. 
131 
SOME PHYSICAL ASPECTS OF GENERAL GEOLOG- 
ICAL CORRELATION, 
BY CHARLES R. KEYES. 
CONTENTS. 
PAGE. 
Introductory 131 
Nature of the problem of geologic correlation 133 
Foundation of geological chronology — 134 
Methods of correlation 136 
General statement . ■ 136 
Chief methods at present in use 137 
Inadequacy of existing methods 189 
Extension of the usual criteria to general application 140 
Main considerations . 140 
Biological relationships 140 
U nconf ormity 144 
Community of genesis 146 
Historical similarity 147 
Physiographic development 148 
Correlation of provinces of dissimilar geological history 149 
Conclusions 152 
INTRODUCTORY. 
The main object of the present communication is to form- 
ulate, briefly, certain results which have been obtained in the 
course of recent attempts to parallel some of the geological 
terranes in the Mississippi valley. The suggestions they offer 
appear to have a much more than local bearing, and to affect 
the stratigraphy of the entire region. They also have an 
important influence upon the whole problem of general 
correlation and, perhaps, also, even upon our present system 
of geological classification. 
In the whole domain of natural science, there is perhaps 
nothing that is more striking, through all stages of its consid- 
eration, from beginning to end, than the fact that natural 
phenomena are rarely the outcome of the action of single, simple 
laws. They all originate in so many remote and complex 
processes that those which are really primary and essential in 
character, are often largely or completely obscured by those 
which, though most conspicuous perhaps, are altogether 
