220 Proceedings of the Royal Physical Society. 
With this explanation of the lines on which my investiga- 
tions have been conducted, we may now pass on to the 
consideration of the plants which characterise those divisions 
of the Carboniferous formation, which have already been 
mentioned as occurring in Britain. f 
The fossil plants of the Carboniferous formation as 
developed in Britain, clearly indicate a great two-fold 
division of these rocks into— 
A. Upprrer CARBONIFEROUS. 
B. Lower CARBONIFEROUS. 
Professor Hull has proposed a threefold division, viz.,— 
UPPER CARBONIFEROUS. 
MIDDLE CARBONIFEROUS. 
LOWER CARBONIFEROUS.! 
In his Middle Carboniferous he includes the Gannister 
Beds or Lower Coal-Measures, the Millstone Grit Series, and 
the Yoredale Series. If the Yoredales are the equivalents 
of the Carboniferous Limestone Series of Scotland, which 
I believe they are, there is included in his Middle Carboni- 
ferous certain groups which have no paleontological affinities 
whatever. The Carboniferous Limestone Series shares, in 
common with the Calciferous Sandstone Series (= Mountain 
Limestone of England), a peculiar group of plants, which 
essentially separates them from the rocks lying above them. 
Whereas the flora of the Lower Coal-Measures and Millstone 
Grit belongs to another paleontological group, which shows 
no close connection with the flora of the underlying rocks. 
If any value is to be placed on vegetable paleontology, 
this threefold division of the Carboniferous Rocks is utterly 
untenable, and seems to have been fqunded on physical 
conditions alone—upon conditions undergoing constant 
change and frequently repeating themselves at different 
periods in the same area on higher horizons. Such physical 
conditions are not typical of age, and are therefore valueless 
for correlating periods of time. 
1 See ante, p. 194. 
