1877.] Explorations in Colorado. 85 
tion of the great Uinta uplift and the blending of its vanishing 
primary and accessary displacements with those of the north and 
south range above-mentioned. Much information was also ob- 
tained concerning the distribution of the local drift of that 
region, the extent and geological date of outflows of trap, ete. 
The brackish water beds at the base of the Tertiary series, 
containing the characteristic fossils, were discovered in the valley 
of the Yampah. They are thus shown to be exactly equivalent 
with those, now so well known, in the valley of Bitter Creek, 
Wyoming Territory. These last-named localities were also 
visited at the close of the season’s work, and from the strata of 
this horizon at Black Butt’s Station three new species of Unio 
were obtained, making six clearly distinct species in all, that 
have been obtained, associated together in one stratum, at that 
locality. They are all of either distinctively American types or 
closely related to species now living in American fresh waters. 
They represent, by their affinities, the following living species: 
Unio clavus Lamarck ; U. securis Lea; U. gibbosus Barnes; U. 
metanurus Rafinesque and U. complanatus Solander. They are 
associated in the same stratum with species of the genera Cor- 
bula, Corbicula, Neritina, Viparus, etc., this stratum alternating 
with layers containing Ostrea and Anomia. 
The close affinity of these fossil Unios with species now living 
in the Mississippi River and its tributaries seems plainly suggest- 
ive of the fact that they represent the ancestry of the living 
ones. An interesting series of facts has also been collected, 
showing that some of the so-called American types of Unio 
were introduced in what is now the great Rocky Mountain region, 
as early as the Jurassic period, and that their differentiation had 
become great and clearly defined as early as late Cretaceous and 
early Tertiary times. Other observations suggest the probable 
lines of geographical distribution, during the late geological 
periods, of their evolutional descent, by one or more of which 
they have probably reached the Mississippi River system and cul- 
minated in the numerous and diverse forms that now exist there. 
The work of the past season shows very clearly the harmoni- 
ous relations of the various groups of strata dver vast areas; 
that although there may be a thickening or a thinning out of- 
8 at different points they can all be correlated from the Mis- 
souri River to the Sierra Nevada Basin. The fact also that 
there is no physical or paleontological break in these groups over 
large areas from the Cretaceous to the Middle Tertiary is fully 
established. The transition from marine to brackish-water forms 
