146 
PROCEEDINGS OF THE ACADEMY OP 
Bluflfs, and extending above the mouth of the Big Sioux River, have, until 
recently, been considered of doubtful geological age, on account of the 
paucity of their organic remains. The discovery, during the past year, at Black- 
bird Hills, of dicotyledonous leaves in this formation, allied to the oaks, 
willows, and others of our deciduous forest trees, together with their position 
with regard to other well-known Cretaceous formations, renders the evidence 
quite clear that a large portion of the strata which we have included in 
No. 1 of our vertical section, are of Lower Cretaceous age."^" In ascending the 
Platte Valley, No. 1 is first observed five miles above the mouth of that 
river in the form of an outlier, resting upon the Carboniferous limestone and 
gradually increasing in thickness to the mouth of the Elkhorn; it then covers 
the surface of the country as far as Beaver Creek, a branch of Loup Fork. We 
have already mentioned its exposure by elevation in the valley of Old Woman's 
Creek; and it also forms a belt around the Black Hills, presenting its usually 
variable lithological characters, and for the most part destitute of fossils. No. 
2, 3, 4 and 5 also occur in their order of sequence outward from the Black 
Hills, as is shown by colors on the map. We have therefore arrived at the 
conclusion, that No. 1, as it is revealed from Council BlufFs to the Big Sioux, 
is Lower Cretaceous ; although two or three beds of yellow and ash-colored 
clays, exposed at low water, near Blackbird Hills, may be Upper Jurassic, no 
organic remains having been found in them to fix with certainty their age. 
We also consider a large portion of No. 1 ?, as seen at the mouth of the Judith, 
Lower Cretaceous, though some of the beds are probably Jurassic. Should 
the Jurassic system be found to exist in the Judith country, of which fact 
there is room for very little doubt, we may look for a large development of it 
in the Great Lignite basin, which stretches northward toward the Arctic Sea. 
The discovery during the past year in the Black Hills, of beds containing 
fresh water fossils of the genera Unio, Planorbis, Paludina, &c., in the lower 
portion of Cretaceous formation No. 1, or the upper part of the Jurassic, 
points to the conclusion that there probably are deposits in the West equivalent 
to those of the Wealden of Europe. The existence of this formation in the 
West, was first suggested by Dr. Leidy after an examination of some Saurian 
rem ains discovered by me near the mouth of the Judith river, in the summer of 
1855. At that time the evidence seemed to be conflicting in its character, the 
Mollusc ous fossils appearing more closely allied to Tertiary forms, while the 
Verteb rate remains were related to those of the Wealden periods. 
The fact that several species of shells from the Estuary beds of the Judith 
were included in No. 1 by us, solely on the strength of information furnished by Maj. 
Hawn, who had informed us that he had found in these beds, or rather in still lower 
beds. Ammonites^, Baculites, Ancylocercs, Copiinella, Inoceramus, ^c, an association of 
fossils never met with in older rocks than the lowest Cretaceous. The fossils, however, 
sent us from these lowest beds by Maj. Hawn, proved, on examination, to belong to quite 
distinct genera from those to which he had referred them ; being all Peimian and Carboni- 
ferous types. Unfortunately, they did not come to hand until after our paper was pub- 
lished. 
That the higher beds constituting the type of our No. 1, as seen near the mouth of the 
Big Sioux, on the Missouri, containing numerous well-preserved leaves of unquestionable 
dicotyledonous trees, apparently belonging to the genera Quercvs, Salix, &c., and closely 
resembling existing species of those genera, cannot be Triassic, or even Jurassic, will, 
of course, be understood by geologists. Although we have as yet no conclusive, 
palseontological evidence that any portion of the beds we have provisfionally included 
in No. 1, in the region of the Judith River is Jurassic, still I am inclined to think that some 
of the beds of this system are there exposed by upheaval in several localities. While ex- 
ploring this wild, rugged country in the summer of 1855, 1 often noticed a series of non- 
fossihferous variegated beds of clays and grits, thrust up from beneath the fossiliferus sand- 
stones, which we have called No. 1 ?, with the same lithological characters as the Jurassic 
strata, developed in the Black Hills. I have httle doubt, that they are exposed around 
Little Rocky, Snowy and Girdle Mountains in the same manner as about the Black 
Hills. 
*Maj. Hawn also found the same or similar leaves in this formation in Kansas. 
[June, 
