370 C. A. White—Fossil Faunas and Floras. 
no. doubt caused many extinctions as well as dispersions of its 
faunas, but which produced little or no effect upon at least a 
considerable part of the contemporaneous plant and inverte- 
brate life. Concerning the nature of those changes which thus 
affected vertebrate life our present knowledge is quite limited, 
and much is therefore left to conjecture. The suddenness, how- 
ever, with which the remains of land vertebrates have succes- 
sively appeared within a great unbroken series of intra-conti- 
nental strata seems to make it necessary to conclude that one 
of the results of those changes was the removal from time to 
time of certain physical barriers which previously restricted 
the dispersion of those faunas respectively. 
I have already given my reasons for believing that the sedi- 
mentation of that great series of strata, from the Laramie to 
the Bridger Group inclusive, was at no time everywhere en- 
tirely interrupted, and that there was for the whole time, and 
within the region where that series was deposited, an unbroken 
continuity of invertebrate and plant life. If these conditions 
actually existed we must necessarily conclude that tie Puerco 
and Wasatch mammalian faunas were both suddenly and inde- 
pendently introduced into the region where they are now 
found, from some other region where they previously existed. 
This view of the case shows the impropriety of regarding 
the earliest strata in which the remains of a certain fauna hap- 
pen to be found as indicating the exact time of the ushering 
in of the epoch to which that fauna is assumed to belong. For 
example, it is claimed by those who have studied the verte- 
brate faunas of the Puerco and Laramie groups that they ought 
to be referred to the Cretaceous period; and they are equally 
positive that the Wasatch Group should be referred to the 
Hocene. Now the dinosaurian remains of the Laramie are 
known to range up to the uppermost strata of that group; and 
many of those of the Coryphodonts and other Hocene mam- 
mals have been found in the lowermost strata of the Wasatch. 
So closely do these two fossiliferous horizons come together, 
and so evident is it that sedimentation was continuous from 
the one to the other, we cannot assume that any considerable 
time elapsed between the deposition of the ‘atest strata of the 
one group and that of the earliest strata of the other, notwith- 
standing the intervention of the Puerco fauna in a part of the 
region where both the Wasatch and Laramie Groups are found. 
Here then, we find the remains of Cretaceous dinosaurs, of a 
unique mammalian fauna, and of a distinctively Hocene mam- 
malian fauna, all in an unbroken series of deposits, each occupy- 
ing separate horizons which so closely approximate each other 
that it is evident sufficient time did not elapse during the de- 
position of that portion of the stratigraphical series to have 
