C.—GEOLOGY 65 
reasonable differentiation was given until Brongniart, in 1849, suggested 
that there were three distinct floras in geological time, and named them 
after the dominant types of plants in each, namely :— 
I. Reign of Acrogens . . (Old Red Sandstone) Carboniferous, 
Permian. 
II. Reign of Gymnosperms . ‘Triassic, Jurassic and Cretaceous 
(Wealden). 
III. Reign of Angiosperms or 
Flowering Plants . . Cretaceous (above Wealden) to 
present. 
We now recognise an older flora in Devonian strata, below the Upper 
Series, which has lately yielded several new and important types. The 
recent interest in this older flora was undoubtedly stirred up by Kidston 
and Lang’s monographs on the plants obtained so near to us in Aberdeen, 
namely, Muir of Rhynie or Rhynie, as it has now become known through- 
out the world. As regards actual numbers of species this older Devonian 
flora is poor, but those that are known indicate considerable diversity in 
organisation. Palaeopitys is a type that Hugh Miller °° classed with the 
higher gymnosperms, but Kidston and Lang ?® would incline rather to 
place it with the Pteridosperms. ‘The Upper Devonian genus Callixylon 
has very specialised secondary xylem, and an argument might be made, 
on that accoant, for some ancestral form, with gymnospermous affinities, 
in the lower rocks. At any rate the higher and lower pteridophytes are 
admitted, as also the Psilophytales and the alge and fungi. In other 
words, there is a fairly representative phase of vegetation ; and a phase so 
different from the succeeding phases, that it deserves to rank as a flora of 
the first order. (May I recall that it was thzs flora that Hugh Miller used 
so frequently in his geological arguments combating the ‘ development 
hypothesis ’ as the theory of Evolution was called in his day. The force 
of the argument is now gone, but it was an important feature in moulding 
geological ideas at the time.) How far down the geological scale this 
flora may extend we do not know, but it certainly carries down into the 
rocks of Lower Old Red Sandstone times. 
A yet older flora is found in the early Palzeozoic and even re-Palzeozoic 
rocks. The only relics of it consist of the algz, and the so-called alge, 
‘that abound at certain levels. The algal character of some of these has 
been seriously challenged ; even some of those now accepted as such may 
have to be relegated to other categories, but some at any rate can be 
accepted. Now these are all marine forms, so far as we know, and their 
very simplicity is a barrier to classification. Are they the lower terms, 
as it were, of several floras, or did one marine type persist through these 
prolonged ages? At present we cannot say. From a geological point 
of view they are useful as indicating a type of deposit, for plants can only 
occur in shallow water; and this affords yet another example of the 
use of plants in geological philosophy. 
28 Miller, Footprints of the Creator. 
29 Kidston and Lang, Tvans. Roy. Soc. Edin., vol. liii, p. 415. 
