(Review of decent Geological Literature. 57 
difference of climate than can be accounted for by the difference of 
latitude. In explanation of this it is suggested that the nearness of the 
ocean on both the east and west sides of the southern portion of the 
Laramie area probably contributed much toward producing its equable 
and moist climate, nearly or altogether free from frosts. Farther north 
there is evidence that the Laramie period included a change from a 
warm to a comparatively cold climate. 
All the species of plants that had been authentically described and re- 
corded up to the date of this memoir in the Laramie group, in the Seno- 
nian of the upper Cretaceous, and in the Eocene, are enumerated for 
comparison in a very elaborate table, which fills more than seventy pages, 
including 1,540 species, of which 1,254 are phenogams. Among the 
phenogamous genera in the Laramie group are Sequoia, represented by 
six species; Taxodium, three species; Thuya, two; Sabal, four, and 
thirteen other species of the palm family; Populus, twenty-three; Salix, 
three; Quercus, twenty-three; Corylus, five, including the two species 
now living in the northern United States and Canada; Alnus, two; 
Betula, three; Juglans, eight; Platanus, eight; Ficus, twenty; Laurus, 
six; Cinnamomum, four; Cornus, four; Aralia, four; Rhus, five; Acer,, 
^hree; Sapindus, four; Vitis, five; Rhamnus, twelve; Nelumbium, two; 
Magnolia, six; Fraxinus, two; Diospyros, four; and Viburnum, fifteen 
species. In total the Laramie flora enumerated in this table comprises 
323 species, of which 275 are phenogams, 226 being dicotyledons. 
The author remarks that the flora of the Laramie group furnishes 
evidence of having descended more or less directly from that of the Cre- 
taceous of this continent, and that in many cases the lines of descent can 
be traced through the upper or Senonian beds to those of the Dakota 
group or American Cenomanian. The diversity of floras now existing 
upon the earth's surface has its analogue in the diverse but at least ap- 
proximately contemporaneous floras of past periods through the Tertiary 
and Mesozoic eras, but in gradually diminishing degree, until in the 
Carboniferous period a nearly uniform flora overspread the entire globe. 
But much of the present extraordinary variety in the floras of different 
countries is attributable to the special agency of the successive glacial 
epochs which have occurred since Tertiary time, driving the floras south- 
ward and out on the southern plains to be destroyed on the return of 
warmer climatic influences or compelled to intrench themselves vipon 
the summits of the mountain ranges, while new and constantly varying 
forms became developed to take their places in the lowlands. 
After extensive detailed comparisons of the species tabulated, the au- 
thor decides that the Laramie flora as closely resembles the Senonian as 
it does either the Eocene or the Miocene. 
Portions of Mr. Ward's collections of Laramie plants, made in 1881 and 
1883, were illustrated in the plates accompanying this memoir, which 
present eighty-four new species, not contained in his table, with fifty-six 
that were before known. The localities of their collection are briefly 
noted, but their descriptions and critical comments upon them were not 
