THE NORTHERN PACIFIC ROUTE. 67 
known as Eagle Butte. This white sandstone with a buff layer at 
the bottom is known to geologists by the local name of Colgate sand- 
stone. The lower part contains in places casts of sea weeds and 
marine shells, so that it is believed to represent the sandy shore of an 
ancient sea. It is supposed to be in part equivalent to the Fox Hills 
sandstone of South Dakota. The rocks overlying the Colgate sand- 
stone in this region are all of fresh-water origin. At Eagle Butte the 
sandstone appears to be nearly horizontal, but it rises gently toward 
the southwest and near milepost 7 it is high in the hills, and the shale 
below it appears at railroad level. The hill near milepost 7, known 
as Iron Bluff, is noted for the beauty and abundance of the fossil 
shells that occur in limestone concretions‘ in the dark shale. The 
shells are so perfectly preserved that they retain their pearly luster. 
From the kinds of shells occurring in the shale and from its character 
it is known to be the same as the dark shale that is poorly exposed in 
the river bluffs at Valley City, N. Dak. It is called the Pierre shale 
and is of Upper Cretaceous age. The fossil shells show clearly that 
the sea must have occupied this part of the country when the shale 
was deposited. At that time, instead of rolling prairies across North 
Dakota and eastern Montana, there were rolling waves and abundant 
marine life. 
or by the Indians who inhabited the 
region. The only Frenchman who is 
thought to have seen the upper part of the 
Yellowstone Valley before the time of 
= and Clark was Verandrye, son be- 
tween the years 1738 and ae penetrated 
ty wilderness far to the west of Lake 
Wi Sees and who BE for a long 
ng the mountains in an ineffec- 
tual a to reach the Pacific slope. 
It is said that he reached the headwaters 
of the Missouri and even penetrated as far 
south as the central part of Wyoming, 
where he was so beset by hostile Indians 
that he was forced to return to the east. 
None of the points described by Veran- | i 
drye have been recognized, so the iden- 
tity of the country which he traversed will 
ys 
some of po wonderful geysers and hot 
people to apply the name. 
!The term concretion is applied to 
rounded bodies of rock that are somewhat 
harder and more resistant than the main 
mass of the formation in which they are 
contained and for that reason remain on 
the surface after the rest of the formation 
has decayed. In many places they are 
nearly spherical, but as a rule they are 
irregular in outline, either elongated in 
a mass resembling the trunk of a tree or 
flattened like a disk. 
The material composing concretions 
differs greatly; in sandstone or sandy 
shale it is generally sand, or sand contain- 
The concretions of Iron Bluff are eoeee 
interesting because they are made up 
almost exclusively of fossil shells. It 
seems probable that the shells grew in 
colonies and thus provided the lime of 
might inspire another Holmes to write a 
poem on the chambered nautilus of the 
ancient sea. 
