56 
POPULAR SCIENCE REVIEW. 
and cycads, a few monocotyledons, as palms ; dicotyledons,' as 
the ordinary forest trees ; but it is the different distribution of 
these classes that forms the marked feature of the two divisions. 
In the lower group, the gymnogens and cryptogams are abun- 
dant ; in the upper, the dicotyledons are most numerous, with 
some^ conifers and ferns. This distribution has some relation 
to the floras preceding and succeeding the Cretaceous period. 
Thus the preceding Jurassic flora is composed of ferns, a few 
Equisetacese, some Coniferge, and a great abundance of Cycadese. 
Three-fourths of all the fossil Zamice and half of the Cycadeae, 
known from all the geological formations, are Jurassic. In the 
lower Cretaceous of Wernsdorf and Greenland, especially the latter, 
Heer finds a marked proportion of this family which is scarcely 
represented in the upper. Of the Coniferce the lower Cretaceous 
rocks (Gault) contain the earliest known representative of 
Sequoia ; this genus is interesting, not only as a persistent type 
ranging from the Cretaceous to the present, but from its wide 
distribution at former geological periods, he. the upper Cretaceous 
and Miocene, and being now represented by twc species only (the 
Redwoods), and those restricted to a comparative limited area 
in North America, the S. gigantea (Mammoth tree or Welling- 
tonia) to the western flank of the Sierra Nevada, and S. sem- 
pervirens along the coast range from the bay of Monterey to 
the frontiers of Oregon.* The absence of dicotyledons, the 
presence of a few monocotyledons, and the abundance of the 
Cycadese. approximate the lower Cretaceous to the previous Ju- 
rassic flora. 
On the other hand, the Upper Cretaceous presents a distinct 
assemblage, and affords the first well-marked proof of the intro- 
duction on the earth of a vegetation allied to our fruit and 
forest trees.f Among the ferns are the living genera Gleichenia, 
Lygodium, Asplenium, together with the earlier forms Spheno- 
pteris and Pecopteris; scarcely any Zamise, but some Coniferse, as 
Abietites, Phyllocladus (?) Sequoia, and its near relative Gflypto- 
strobus, now living in China. Of the numerous genera of dicoty- 
ledons are the fig, willow, poplar, beech, oak, plane, sweet -gum- 
tree (Liquidambar), sassafras (abundant in the Dakota group), 
Magnolia, Liriodendron (tulip-tree), some Proteacese, and other 
living and extinct genera, including Gredneria , many of them 
having at present a very different geographical distribution. 
Among others, the Magnolia and Liriodendron belong to North 
America by origin, succession, and presence. Of the eight species 
* See Lesquereux, “ Cretaceous Flora,” p. 116 ; and Professor A. Gray, 
u Address to the American Association at Dubueque, Iowa,” Aug. 1872. 
t Lesquereux remarks that “ the essential types of our actual flora are 
marked in the Cretaceous period, and have come to us after passing, without 
notable changes, through the Tertiary formations of our continent.” 
