138 
IOWA ACADEMY OP SCIENCES. 
and are, really, the foundation 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 difficulties 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 difficulties 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 events, 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 weigh 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. 
*Iowa Geol. Surv., Vol. II, p. 63, 1893. 
