138 IOWA ACADEMY OF SCIENCES. 



and are, really, the foimdation of our commonly accepted sys- 

 tem of geological synchrony. However, it is beginning to be 

 recognized more and more clearly that organic remains are not 

 the all-deciding factors in questions of correlation, that they 

 are, in fact, merely accidental characters, and that when 

 depended upon they must always be taken in connection with 

 physical features. In actual practice they are regarded as 

 corroborative evidence after the main points of the special 

 problem under consideration have been determined by other 

 means. 



In the recognition of these difticulties it was recently stated" 

 that all the principal characters, stratigraphical, lithological 

 and faunal, of every formation, were so intimately interrelated 

 in origin that the proper interpretation of any one of the three 

 classes of phenomena presented should, under normal condi- 

 tions, indicate the more salient features of the other two, 

 but that, ordinarily, great difticulties were encountered in 

 attempting to infer the entire geological history of a series of 

 beds from a single group of facts. It was fully appreciated 

 that the geological records were very imperfect, but at the 

 same time they were not believed to be nearly so fragmentary as 

 generally supposed, though the larger part was, in a great 

 measure, more or less inaccessible; those portions of the 

 lithosphere that were open to investigation were as yet only 

 partially considered. For a long time to come, the territory 

 open to inspection would require constant study before the 

 history could be made even measurably complete. Never- 

 theless, at the present time, it was considered absolutely 

 necessary to carry on investigations, involving the historical 

 sequence of geological eveuts, along all three lines at once, 

 every fact being needed to throw light upon the general 

 scheme. If the problems were attacked in any one of the three 

 directions alone, without due regard for the evidence presented 

 by the others, very different, and perhaps antagonistic, 

 conclusions might be reached, at least in the present state 

 of knowledge. In the interpretation, then, of the geological 

 history of a region, and in the erection of a classification of 

 the formations in accordance with that interpretation, it 

 is of prime importance to w^igh carefully all the evidence 

 set forth by the arrangement, composition and contained 

 organic remains of the rock series as a whole, and of its 

 several parts regarded as distinct units. 



*Iowd, Geol. Surv., Vol II, p. 62, 1893. 



