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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 superposition 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 parallel! Qg 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 
