IOWA ACADEMY OF SCIENCES. 131 



SOME PHYSICAL ASPECTS OF GENERAL GEOLOG- 

 ICAL CORRELATION, 



BY CHARLES R. KEYES. 

 CONTENTS. 



T ^ ^ * 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 I37 



Inadequacy of existing methods 139 



Extension of the usual criteria to general application 140 



Main considerations 140 



Biological relationships 140 



Unconformity I44 



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, brieily, 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 



