Geologie Correlation.— Ward. 45 
been the basis of my remarks upon the method pursued, I have 
here shown the number in each horizon respectively, of ferns, 
equiseta, rhizocarps, cycads, and conifers, these being the only 
types represented in the foreign distribution of that flora. 
The method of reasoning with regard to the age of the forma- 
tion from data of this kind, is important to be considered. The 
usual way is to prepare only some such extended table as the first 
one described, often without taking account of related species, 
and then to proceed to discuss each species and its bearings upon 
the age of the group. The data thus considered are only abso- 
lute, not relative, and conclusions drawn from them are apt to be 
very misleading. I have frequently pointed out that the great 
mistakes of Heer and Lesquereux in placing the American plant- 
bearing strata too high in the geological scale was due to this 
fallacy. These geologists compared the Dakota group flora, the 
Laramie group flora, and all the higher floras of the United 
States with those of the Miocene of Europe and of the Tertiary 
of the polar regions. They laid great stress on the fact that 
species were found in these formations which could not be dis- 
tinguished from American species, or which could scarcely be so 
distinguished. Such an argument is of little value in view of the 
immense magnitude of the Tertiary floras considered. The Ter- 
tiary flora of Europe embraces elements of antecedent floras, and 
it is so well preserved that the number of these pre-Tertiary 
forms, holding over into the Tertiary, is far greater than the 
number of pre-Tertiary forms that have thus far been found in 
lower formations, which have yielded comparatively few plants 
A comparison, therefore, of the American Laramie group, for 
example, with the European Tertiary alone without considering 
the European Cretaceous flora, and without noting this continu- 
ance of Cretaceous types into Tertiary time, proved to be ex- 
tremely misleading, and resulted in the general impression, which 
still prevails in Europe, that our Laramie group is of Tertiary 
age. Perceiving this fallacy | was the first, and so far as | know, 
am the only one, to attempt an enumeration of the Cretaceous 
species of fossil plants with a view to their comparison with those 
of the Laramie group of the United States. * 
The system which I have just described obviates this fallacy. 
*Synopsis of the Flora of the Laramie Group. Sixth Ann. Rept. of 
the U.S. Geol, Surv., pp. 445-514. 
