40 The American Geologist. January, 1892 
that which is of geological importance, is that the form in ques- 
tion be distinctly recognized, that it be carefully portrayed, and 
that what has been found be characterized in accurate descriptive 
language and represented by clear and careful delineation. There 
must be no doubt when the same form is seen again at a different 
locality, as to whether it is really the same form. This is a vital 
point with the geologist. If the form, no matter what it may 
really be, is something clear and distinct, which can be recognized 
when seen anywhere, and if it is characteristic of a given horizon 
or locality, it becomes to that extent of value in fixing the rela- 
tive age of any other deposit in which it may be found. If only 
found in two localities or at two points on the earth's surface the 
deduction, though not absolute, is legitimate that unless there is 
evidence to the contrary the two localities are of somewhat simi- 
lar geologic age. But if the object be very abundant, and char- 
acteristic of some well known group or horizon, then it is that it 
becomes of great importance as a characteristic fossil, independ- 
ently of how much may be known of its true botanical nature. 
Il. Mernops. 
I propose next to indicate the general method which I have 
adopted in the application of these principles to geologic correla- 
tion by the aid of fossil plants. 
In a broad sense this, of course, consists in the comparison of 
similar floras, and the conclusion from them of similarity of age; 
but there are many limiting circumstances to be taken into the 
account. If the localities at which similar floras occur are not 
widely separated geographically, the conclusion of similarity of 
age is more or less reliable. For example, when we find that the 
flora of the Richmond coalfield is very similar to that of the North 
Carolina coalfield, the inference that these two coalfields are of 
similar age is wholly legitimate. And even when we find the 
same species, to a considerable extent, in the Triassic of New 
Jersey and in that of Connecticut and Massachusetts as occurring 
in Virginia and North Carolina, the inference cannot be very 
wide of the mark that the strata containing these plants were de- 
posited at about the same time from Massachusetts to North Carolina, 
In proportion as these similar floras are separated geographi- 
cally the inference of the similarity of age and deposition grows 
weaker, but it will remain strong as long as the two localities are 
on the same continent, or as evidence exists that an unbroken sea 
