96 Iron Butte, Montana. — Calvin. 
horizon. Cross either valley, to the east or west, and you find 
the buttes carved in the sandstones and shales of the Tertiary, 
while Iron Butte is made up wholly of deposits belonging 
to the Ft. Pierre or Fox Hills group of the Cretaceous. 
On the west side of Cedar creek the Tertiary strata dip at 
quite a high angle to the westward ; to the east of Sand creek 
they are very nearly horizontal. A strong fold in the strata 
giving the sharp dip toward the west, with probably a slight 
fault along Sand creek valley would account for the interest- 
ing relations of the Cretaceous deposits exhibited at Iron 
Butte. 
The fossils of Iron Butte are especially interesting on account 
of the fact that species elsewhere characteristic of the Ft. 
Pierre and Fox Hills groups respectively are here commingled 
in such a way as to indicate that both faunas lived together 
on amicable terms and were perfectly at home in the Creta- 
ceous seas occupying that particular region. Professor Meek 
in his great work on "Invertebrate Paleontology," Gov't Print- 
ing office, 1876, refers frequently to a locality on the "Yellow- 
stone river, one hundred and fifty miles from its mouth, where 
it (the species he is describing) occurs in beds containing a 
blending of the fossils of the Ft. Pierre and Fox Hills groups 
of the upper Missouri Cretaceous series." All the species thus 
described by Meek occur at Iron Butte, and as the distance by 
river is about one hundred and fifty miles from the mouth of 
the Yellowstone, there is reason to believe that Iron Butte is 
indeed the locality from which Meek's specimens were obtained. 
Geologists may be interested in having this somewhat remark- 
able exposure more definitely located than was possible at the 
time Meek wrote. 
The fossils at Iron Butte are not distributed generally 
through the deposits, but occur in hard, irregularly shaped 
concretionary masses that break with a splintery fracture into 
sharp angular fragments. These concretions may be from a 
few inches to several feet in diameter. Sometimes they seem 
to be disposed in layers, but very frequently it is impossible 
to discover any order of arrangement. 
Two or more distinct assemblages of fossils occur at Iron 
Butte, but the matter needs further investigation. For exam- 
ple the Scaphites 7iodosus var. plenus and the S. nodosus var. 
quadrangular is do not occur together. Moreover each variety 
