the Eastern margins of the Rocky mountains, 323 
might be used for the same purposes, The two characteristic 
species of fossils of this division are found everywhere, Ostrea 
y congesta and Inoceramus problematicus. All along the slope 
of the mountains No, 3 still retains its chalky nature but 
becomes quite shaly, none of the layers ever becoming more 
: than one or two inches in thickness, Thisis the case at the 
sources of the Missouri along the Big Horn and Wind river 
mountains also, from the South Pass to Pike’s Peak, and on the 
western slope wherever this bed is exposed. Near Denver, at 
Marshall’s coal mine, No. 3 has been changed by heat into a 
grayish compact limestone, quite hard and brittle in its frac- 
ture, which makes an excellent flux in smelting ores, But this 
change is local, for 16 miles north of this point it presents the 
same laminated character. It seems that No. 3 loses its mas- 
sive chalky character, by which it first attracted attention on the 
Missouri river, in its westward extension, so that along the 
margins of the mountains except in one locality it cannot prove 
of any economical value, while between 98° and 100° longitude 
it becomes very useful not only for lime but also for build- 
any well defined paleontological proof of its existence. Near 
ort Benton are a series of Cretaceous beds containing some 
seams of impure lignite, and numerous species of fossils, not one 
of which is identical with those so abundant in Nos. 4 and 5 
lower down on the Missouri. These beds have been placed 
provisionally in the general section as a portion of No. 1, but 
the region about Fort Benton needs a more careful examination 
before any positive conclusions can be arrived at. Around the 
Black Hills isa bed of ive sili rocks, some layers form 
ing a pudding stone which in some localities takes the name of 
fortification rocks. These hold a position between No. 2 Cre- 
taceous and the Jurassic marls. The same are seen along the 
margin of the Big Horn mountains, in which I observed a 
bed of impure lignite, an abundance of silicified wood, and 
some uncharacteristic Saurian bones. From the Wind River 
mountains to Pike’s Peak, these same siliceous and pebble ce- 
mented rocks occur holding the same geological position, form- _ 
Ing, as it were, beds of transition between the Cretaceous and 
