134 
IOWA ACADEMY OF SCIENCES. 
admitted, but in each part the details, and perhaps some of 
the characteristic larger features, are very different. The 
whole history is made up of the histories of the parts, of the 
nations, of the provinces. In a similar way, geological history 
reaches back into a haze of distance, compared with which the 
beginnings of human history are but as a moment ago. As 
the history of man is a history of nations and dynasties, more 
or less intricately related, so also is geological history a history 
of parts, ■ of provinces, overlapped, interwoven, merged into 
one another, but each retaining, more or less distinctly, its 
identity, thrusting out its idiosyncrasies, and presenting the 
evidence of its relatious with its neighbors. 
The development of geological provinces has another 
parallel in the progress of nations. Some great events have 
been recorded in the history of all; others in only a few. At 
certain periods a mingling, an absorption, or a complete 
effacement of some parts has taken place; at other times has 
occurred conquest and expansion. 
Could the events of all nations that have ever existed be 
arranged on a chart, or in tabular form, so that those concern- 
ing each could be brought together in a vertical column, 
and so that the different columns would stand side by side 
in their proper positions in the time scale, with its major sub- 
divisions marked off by horizontal lines, it would be found 
that at certain times great events would affect several and 
perhaps many nations, and that at such times similar events 
would affect different groups of nations, or part of one group 
and part of another. 
In the same manner are the events of geology recorded 
in different parts of the earth. While the general sequence is 
similar everywhere perhaps, great changes affect the different 
parts in different ways and with varying intensity. So, in tab 
ulating the geological events of different provinces, the 
standard corresponding to the time scale in human history 
must be absolute and far-reaching, and not changeable and 
local. The determination of such a standard is the one great 
problem of stratigraphy. 
THE FOUNDATION OP GEOLOGIC CORRELATION. 
In the correlation or comparison of geological terranes, 
experience has shown that the subject may be viewed from 
at least four very different points of vantage. The aspect 
