250 Prof. Hayden's Explorations. [April, 
district assigned to him was 40, and the entire area was 
about 3000 square miles. Barometric observations were 
made whenever needed, and about 2000 angles of elevation 
and depression with fore- and back-sights, so that material 
for obtaining correct altitudes is abundant. 
The rocks of this district embrace all the sedimentary for- 
mations yet recognised by the investigators who have studied 
the region that lies between the Park Range and the Great 
Salt lake, namely, from the Uinta quartzite (which underlies 
the carboniferous) to the Brown’s Park group, or latest ter- 
tiary inclusive. Not only has the geographical distribution 
of these formations been mapped, but all the displacements 
of the strata have been traced and delineated. The last- 
named investigations bring out some interesting and im- 
portant faCts in relation to the orthographic geology of the 
region, especially as regards the eastern termination of the 
great Uinta uplift, and the blending of its vanishing primary 
and accessory displacements with those of the north and 
south range above mentioned. Much information was also 
obtained concerning the distribution of the local drift of that 
region, the extent and geological date of outflow of trap, &c. 
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 lo- 
calities were also visited at the close of the season’s work, 
and from the strata of this horizon at Black Buttes 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 spe- 
cies now living in American fresh waters. They represent 
by their affinities the following living species ; — Unio clavus, 
Lamarck : IJ. securis, Lea ; U. gibbosns , Barnes ; U. meta- 
neorus , Rafinesque ; and U. complanatus, Solander. They 
are associated in the same stratum with species of the 
genera Corbulo, Corbicula, Neritina, Viriparus, &c., and 
which stratum alternates 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 suggestive of the fact that they represent the ances- 
try 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 
