CHAPTER VI. 
THE BIONIC VALUE OF FOSSILS. 
GENERAL STATEMENT. 
The essential difference between the three classes of evidence upon 
which geologists base their determinations of equivalency of com- 
pared formations having been demonstrated, a few words may be 
said regarding the nature of the evidence . by which fossils record 
definite epochs of geological time. 
Uniformity in rock constitution we all understand, and it requires no 
special analysis. Stratigraphical equivalency is readily perceived to 
be based upon structural uniformity; and in describing two formations 
as stratigraphical equivalents we mean that they are the same strue- 
tural parts of the earth's crust. In making determinations of faunal 
equivalency, however, the presence of one or several fossils is not 
sufficient to establish close correlation, for the reason that the same 
fossil species may occur throughout many feet of thickness of sedi- 
ments, and anywhere in that range may exhibit the same fossil forms. 
It becomes necessary to deal with the aggregate fauna regarding 
which the modifications are constantly taking place. Not only must 
we treat of fossils as aggregates, but we must have some means of 
measuring the aggregates other than the scientific names of the fossils. 
While their names are essential and cover a great many particulars, 
in order to extract the evidences of time we must be able to deal spe- 
cifically with those elements which are associated directly with the 
passage of time. 
In the previous pages I have referred to the bionic values of fossils, 
and have arrayed a mass of statistics, gathered and formulated in 
such ways as to exhibit these bionic relations, and the reader will now 
be ready to consider more particularly what is the nature of this 
special method of treatment of fossils as evidence of passage of time. 
Fossils, as morphological records of the living organisms of the past, 
are of inestimable value in reading the history not only of the organ- 
isms themselves, but of the conditions of the environment through 
which they struggled and to which they were adjusted. But form 
such as the fossil expresses, and in general such as is expressed by 
the hard parts of all organisms, is extremely complex. It is impos- 
sible to describe it in geometrical terms, as may be done in the case 
of minerals. Although descriptions of form may be given which will 
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