2 ry . . 
ob The American Geologist. January, 1892 
I propose, in this paper, to first set forth a few principles which, 
as it seems to me, should govern the study of paleobotany as an 
aid to geologic correlation, and then to explain the methods which 
I have adopted for their application. 
I. PRINCIPLEs. 
Bearing in mind the law of homotaxis, and not forgetting that 
similar floras may have flourished in different parts of the world 
at different times, it. nevertheless still remains true that the oc- 
currence of similar floras in different parts of the world, has a 
strong bearing upon the question of the age of the strata in 
which these floras occur. That is to say, although these floras 
may have flotrished at different times, the difference between the 
epochs at which they grew cannot be very great, and while an 
exact identity of age cannot be predicated, still it is safe to say 
that deposits containing similar floras must have been laid down 
at no great distance apart chronologically speaking. 
The great types of vegetation are characteristic of the great 
epochs in geology, and it is impossible for types of one epoch to 
occur in another. For example: It frequently happens in a region 
which is much broken up that the stratigraphical geologist is 
greatly puzzled to determine the relative position of certain rocks, 
The time has gone by when geologists rely implicitly upon the 
appearance of the rock in determining age, and rocks of Carbon- 
iferous age may have so close a resemblance to those of Tertiary 
age that it is impossible to distinguish them lithologically. In 
such cases a single characteristic fossil found in place is sufficient 
to settle the question. The fossil may be a mere fragment not 
specifically determinable, but if its reference to a great type of 
vegetation is certain this is as conclusive as if it were known to 
what species it belonged. For example, a dicotyledonous leaf 
from a stratum supposed to be Carboniferous, enables the paleo- 
botanist to say with absolute certainty that such a reference is 
impossible. On the other hand a single sear of Lepidodendron 
or Sigillaria from a deposit supposed to be Tertiary or Mesozoic 
is equally conclusive. It may be said that such cases are not 
common, but [ have had in my limited experience a number of 
instances of precisely this nature where thoroughly competent 
geologists were much perplexed, and were set right by such a 
single fact. 
