106 United States Survey of the Territories. (February, 
but both the Green River and Bridger groups also are well devel- 
oped, each possessing all its peculiar and usual characteristics as 
seen at the typical localities in the great Green River basin, north 
of the Uintah mountains. This, added to the known existence of 
Bridger strata in White River valley, and the extensive area occu- 
pied by the Green River group between White and Grand rivers, 
has added very largely to our knowledge of the southward exten- 
sion of those formations. 
In all the comparative examinations of the formations or groups 
of strata that have just been indicated, he has paid especial atten- 
tion to their boundaries, or planes of demarkation, crossing and 
recrossing them wherever opportunity offered, noting carefully 
every change of both lithological and palzontological characters. 
While he has been able to recognize with satisfactory clearness 
the three principal groups of Cretaceous strata, namely, the 
Dakota, Colorado, and Fox hills, on both sides of the Rocky and 
Uintah mountains, respectively, they evidently constitute an un- 
broken series, so far as their origin by continuous sedimentation 
is concerned, While each of the groups possesses its own pecu- 
cliar paleontological characteristics, it is also true that certain spe- 
cies pass beyond the recognized boundaries of each within the — 
series. 
The stratigraphical plane of demarkation between the Fox hills, 
the uppermost of the undoubted Cretaceous groups, and the Lara- 3 
mie group, the so-called Post-Cretaceous, is equally obscure; but — 
the two groups are palzontologically very distinct, inasmuch as 
the former is of marine origin, while the latter, so far as is now 
known, contains only brackish-water and fresh-water invertebrate — 
forms. He reports a similar obscurity, or absence of a strati- 
graphical plane of demarkation, between the Laramie and Wasatch 
groups, although it is there that the final change from brackish to 
entirely fresh water took place over that great region. Further- 
more, he finds that while the three principal groups of the fresh- 
water Tertiary series west of the Rocky mountains, namely, the — 
Wasatch, Green river, and Bridger groups, have each peculiar 
characteristics, and are recognizable with satisfactory distinctness _ 
as general divisions, they really constitute a continuous series of 
strata, not separated by sharply-defined planes of demarkati 
either stratigraphical or palæontological. 
Messrs. S. H. Scudder, of Cambridge, and F. C. Bowditch, of 
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