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 relations 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 OF 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 



