IOWA ACADEMY OF SCIENCES. 133 



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 prove 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 OF 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 



