THE PRIMITIVE VEGETATION OF THE EARTH. 415 
higher cryptogams, represented by Ferns, Club-mosses, and 
Equiseta, share the world with the Gymnosperms, repre- 
sented by the pines and Cycads, while the higher phæno- 
gams on the one hand, and lower cryptogams on the other, 
are excluded. Hence, the Mesozoic age has been called that 
of Gymnosperms, while the Paleozoic is that of Acrogens. 
These names are not, however, absolutely accurate, as we 
shall see that one of the highest forms of modern vegetation 
can be traced back into the Devonian; though the terms are 
undoubtedly useful, as indicating the prevalence of the types 
above mentioned, in a degree not now observed, and a cor- 
responding rarity of those forms which constitute our preva- 
lent modern vegetation. 
It is my present object shortly to sketch the more recent 
facts of Devonian and Upper Silurian Botany, and to refer 
to a few of the general truths which they teach. The rocks 
called Devonian in Europe being on the horizon of the Erie 
division of the American geologists, which are much more 
fully developed than their representatives on the Eastern 
Continent, I shall use the term Hrian as equivalent to De- 
vonian, understanding by both that long and important 
geological age intervening between the close of the Upper 
Silurian and the beginning of the Carboniferous. 
Just as in Europe the rocks of this period present a two- 
fold aspect, being in some places of the character of a de- 
posit of "Old Red Sandstone," and in others indicating 
deeper water, or more properly marine conditions, so in 
America, on a greater scale, they have two characters of 
development. In the great and typical Erian area, extend- 
ing for seven hundred miles to the westward of the Apala- 
chian chain of mountains, these rocks, sometimes attaining 
to a thickness of fifteen thousand feet, include extensive 
marine deposits; and except in their north-eastern border 
are not rich in fossil plants. In the smaller north-eastern 
area, on the other hand, lying to the eastward of the Apala- 
chian range, they consist wholly of sandstones and shales, 
