the Eastern margins of the Rocky mountains. 325 
ous rocks are mostly of a red arenaceous character, with a few 
layers 2 to 10 feet in thickness of whitish or yellowish limestone. 
rom these limestones I collected Productus Prattenianus, 
Athyris subtilita and other well known Carboniferous forms. 
Above these red beds which contain intercalated layers of lime- 
stone is a considerable thickness of purely red arenaceous beds, 
but in studying all these rocks with some care from Pole creek 
nearly to Pike’s Peak, I could not separate the red beds from’ 
Black Hills, Big Horn and Wind River mountains. Near the 
Red Buttes there is a bed of siliceous pudding stone resting 
on the metamorphic rocks which may be the Potsdam in its 
southern extension, but south of Fort Laramie to Pike’s Peak, 
it is somewhat dou btful whether any trace of it exists. If it 
occurs at all it is a very thin layer, for the most part concealed. 
So far as I could determine, the Carboniferous rocks rest directly 
(though not conforming) upon the pe nape na rocks. Shee 
see true nite. 
The above remarks, founded on observations that have been 
big and hope: They me seem to reach their culmination a 
far from the central portion of the ee area y 
Missouri, and lose to a great extent their distinctive a 
ters beyond its limits. 
2nd. The Potsdam sandstone and the Jurassic beds Agar 
more remarkable changes than any of the others. 
ee 3 — formations are well marked, both Tithologically and 
tologically ; in their southward extension they oak iE 
dada: a so that south of Fort Laramie to Pike’s 7 
_ becomes a matter of doubt whether they exist at ‘i | e 
e inference therefore is that these groups of rocks are not \ lt : 
_ defined, if they occur at all south of the Arkansas. In 
Of this statement is the fact that although this southern region 
