Or<>f((.i-is : A Method of Correhifion. — Kcjii's. 291 
the older rock masses. Of late the bringing in oi' the geo- 
graphic methods as an aid to geologic interpretation finds a 
number of excellent expressions, but none happier nor more 
forceable than McGee's consideration of the Coastal i)lain de- 
posits of the Atlantic slope. *- 
It is in the extension of Irving's theme a.>^ outlined, under 
the guidance of modern physiographic interpretation, that 
stratigraphy is believed to have found a rational and practical 
method of correlation and classification that, in its funda- 
mental concepts, is entirely independent of the usual and al- 
mo.st universal paleontologic standard. 
The criteria which have been employed at different times 
in the correlation of geological formations have been found to 
have widely varient values. But since all systematic arrange- 
ments of sedimentary deposits have for an ultimate end the 
delineation of the real superposition and relative ages of all 
strata, it is manifest that the scheme incorporating in its plan 
the actual sequence of the processes which have produced the 
events is the one which most nearly meets the requirements 
of a rational foundation for geological chronology. In pro- 
portion therefore as a classification is genetic is it of value as 
epitomizing the history of a region. 
Since the recognition, at the beginning of the eighteenth 
century, of the geological significance of the bedded charac- 
ter of nearly every portion of that part of the lithosphere 
open to observation, the normal order of superposition of the 
layers has formed one of the chief problems of stratigraphy. 
In a single rock exposure it is ordinarily eas}' to determine 
which beds were laid down first and which last. Kut in mak- 
ing a comparison of two sections which are not visibly con- 
nected the case is not so simple, and when the two sections 
are widely separated the diHIculty of paralleling them is cor- 
respondingl}' increased, and exact correlation is perhaps out 
of the question. It is the special i)rovince of geological cor- 
relation to establish a general chronological se(|uence of rock 
successions more or less widely' separated. In tin- past the 
standards for this determination have been numerous. As 
they have cojue to be tested practically in the field they have 
been, one bj'^ one, abandoned entirely, passed over, or in lieu 
*U. S. 0.e<)l. Surv.. 12th .\nn. Kept.. ))]). 245 520. 1891. 
