CHAPTER I. 
THE PRINCIPLES OF CORRELATION. 
IMPORTANCE OF CORRELATION. 
In the Ninth Annual Report of the United States Geological Survey 
(1889), the Director called attention to the importance of correlation 
in the work of the Survey. His words are: 
In order to develop the geological history of the United States as a consistent 
whole, it is necessary to correlate the various local elements. . . It is especially 
important to determine the synchrony of deposits. So far as the outcrops of strata 
can be continuously traced, or can be observed at short intervals, correlation can 
be effected by the study of stratigraphy alone. The correlation of strata sepa- 
rated by wide intervals of discontinuity can be effected only through the study of 
their contained fossils. This is not always easy, and it is now generally recog- 
nized that it is possible only within restricted limits. As distance increases the 
refinement in detail of correlation diminishes. 
Recent discussions in connection with the work of the International Congress 
of Geologists have shown that different students assign different limits to the pos- 
sibilities of correlation and give different weights to the various kinds of paleon- 
tologic evidence employed. 
The study of the data and principles of correlation is thus seen to be a necessary 
part of the work of the Geological Survey/' 
CORRELATION DIVISION OF THE U. S. GEOLOGICAL SURVEY. 
A division of the Survey was thereupon established for the purpose 
of preparing essays on correlation, and summarizing existing knowl- 
edge bearing on the correlation of American strata. A number of 
essays were subsequently prepared by specialists and published as 
bulletins of the Survey. Those now published are as follows: 
No. 80. Devonian and Carboniferous, H. S. Williams, 1891. 
No. 81. Cambrian, C. D. Walcott, 1891. 
No. 82. Cretaceous, C. A. White. 1891. 
No. 83. Eocene, W. B. Clark, 1891. 
No. 84. Neocene, Dall and Harris, 1892. 
No. 85. Newark, I. C. Russell, 1892. 
No. 86. Archean and Algonkian, C. R. Van Hise, 1892. 
This attempt to bring together the facts available for the correla- 
tion of American formations was a direct consequence of the work of 
the International Congress of Geologists, and particularly of the 
American committee of the congress whose report was made to the 
London session of the congress in the year 1888. 
"Ninth Ann. Rept. U. S. Geol. Survey, 1889, p. 10. 
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