IOWA ACADEMY OP SCIENCES. 
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first upon some one particular phase or striking feature, 
and then on another, are not entirely erroneous, for the 
reason that they represent some special workings of funda- 
mental laws that are not, and cannot be, always discerned, 
until greater advancement in general knowledge has been 
made. In this respect, they partake of the nature of working 
hypotheses. A long time may be required to pro ve their faults, . 
and then new schemes arise. In practice, then, the establish- 
ment of a rational system of geological chronology, or classi- 
fication, is not to be sought in the comparison of any one set of 
external features, but rather in the direct causes or processes 
which have given rise to the phenomena. The final outcome 
is reached by a comparison of all the groups of data relating 
to the physical history as a whole. 
NATURE OF THE PROBLEM OP GEOLOGICAL CORRELATION. 
Regarding as the main function of geological correlation, 
the establishment of a practical scale of stratigraphic succes- 
sion, to which may be referred all geological terranes, the 
critical critera adopted become essentially the basis of geolog- 
ical classification or of historical geology. Moreover, a 
rational classification of geological phenomena reflects the 
genesis of the events recorded, and this is manifestly the 
ultimate aim of all methods of paralleling strata. 
It is a favorite simile of geologists to liken the progress of 
geological events to human history. But they stop short of 
the most important step of all in not making the comparison 
full and symmetrical. In the history of mankind, there is in 
the time units, the year, the decade and the century, an abso- 
lute scale for gauging all events. In developing geological 
history, this standard of comparison, of course, fails, because 
of the inapplicability of our ordinary units of time, and with 
this failure, no attempt is made to carry out the all-important 
idea that is fundamental in human history, and look for some 
other unit that is, in its nature not comparative, not variable, 
not local in application, but fixed and independent of any 
inherent character. 
As human history is traced backward, the clear coloration 
of the present gradually fades with time, until lost in the haze 
of distance and uncertainty, tradition and myth. That the 
growth and progress of the races of mankind have been much 
the same in all the various parts of the world, is generally 
