136 IOWA ACADEMY OF SCIENCES. 



fundamental laws. Only in this way can the main object, the 

 establishment of an adequate and elastic system of geological 

 correlation, be attained, and a ready interpretation of the his- 

 tory of terrestrial phenomena be made. Since, from the 

 strata of the globe must be deciphered the records of its 

 history, the leading facts to be ever borne in mind and to be 

 recognized to their fullest possible extent, are that the 

 elements of sedimentation are in large part the products of 

 land decay, which form seaward-creeping fringes around the 

 continental masses, and that the cessation of the action of the 

 processes favorable is one of the prime factors in beginning 

 each new cycle, or great epoch, in the physical history. 



SOME METHODS OF GEOLOGICAL CORRELATION. 



General Statement. — In the present connection it is unneces- 

 sary to enter into details regarding all of the various stand- 

 ards of correlation that have been proposed. As all 

 systematic arrangements of sedimentary deposits have for an 

 ultimate end the real determination of the superposition or 

 relative succession of all strata, it is manifest, from what has 

 already been said, that the scheme incorporating in its plan 

 the actual sequence of the processes that have produced the 

 events, is the one which most nearly meets the requirements 

 of a rational foundation for geological chronology. In propor- 

 tion, therefore, as a classification is genetic, it is of value as 

 epitomizing the history of a region. 



From the time when the real significance of the bedded 

 character of nearly every portion of the lithosphere open to 

 observation first came to be recognized, at the beginning of 

 the eighteenth century, the normal order of suj)erposition and 

 the equivalency of the layers has formed one of the chief 

 problems of stratigraphy. In a single rock exposure it is, 

 ordinarily, easy to determine which beds were laid down first 

 and which last. However, in making a comparison of two sec- 

 tions which are not visibly connected, the case is not so 

 simple; and when the two sections are widely separated, the 

 difficulty of paralleling them is correspondingly increased, 

 and exact correlation, perhaps, finally becomes entirely out of 

 the question. It is the special province of geological correla- 

 tion to establish a general chronological sequence of all rock 

 successions, particularly those more or less widely separated. 

 In the past, the standards for this determination have been 



