Orofd.ci.s : A Jfcf/io</ of Correlation. — k'c/i's. "i'.).} 
tive evidence after the main points have been determineil by 
other means. For this reason chiefly it was set forth recent- 
ly* that all the principal characters, stratigraphical, litholog- 
ieal and faunal, of every formation were so intimately inter- 
related in origin that the proper interpretation of any one of 
the three classes of phenomena presented should, under normal 
conditions, indicate the more salient features of the other two; 
but that, ordinarily, great difficulties were encountered in at- 
tempting to infer the entire geological history of a series of 
beds from a single group of facts. The larger part of the 
preserved records is in great measure inaccessible. Those 
portions which are open to investigation have been as yet onh' 
partially considered. For a long time to come the territory 
open to inspection will require constant study before the his- 
tory can be made out even measurably complete. At the 
present time, therefore, it becomes absolutely necessary to 
carry on investigations, involving the historical sequence of 
geological events, along all three lines at once. Every fact is 
needed to throw light upon the general theme. If the prob- 
lems were attacked in any of the three directions alone, with- 
out 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 in- 
terpretation, then, of the geological history of a region, and 
in the erection of a classification of the formation in accord- 
ance 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. 
It has already been intimated that the basis of geological 
classification has been at various times in accordance with the 
very different standards, and that these have (iontinualU' 
changed. In passing from one to another, however, the 
change has been gradual and not abrupt; for being bound so 
inseparably tf> the past it is well-nigh impossible for us to at 
once cast aside old ideas, even after we are fully convinced of 
their untrustworthiness. So, in clothing new conceptions in 
words, we unconsciously and unavoidably incorporate state- 
ments that are not only deceptive but which have their founda- 
*Io\va Geo!. Siirv., vol. ri. p. 62. 189,3. 
