154 Association of American Geologists and Naturalists. 
A. Argillaceous limestone, containing Inoceramus barbarini very 
much compressed ; three feet of this rock appears above the water ; 
how deep it extends below he had no means of ascertaining. Dissem- 
inated through it is iron pyrites in great abundance. 
B. A calcareous marl, generally from thirty to forty feet thick, con- 
taining few fossils; an Orbicula and fish scale were found in it. 
C. A ferruginous clay containing selenite in acicular crystals, and of 
great variety and beauty of form. 
These rocks, which are always found thus associated, constitute the 
base of the cretaceous formation of the Upper Missouri; it extends from 
the Sioux to the Ayoway River, twelve miles, and rests immediately on 
the carboniferous limestone—all the intermediate strata appear to be 
wanting. Along this line, A disappears on account of the rise of the 
country, and between B and C is found a thin layer of fibrous carbo- 
nate of lime, containg Ostrea congesta of Conrad. The specimens of 
marl sent from this place have been found by Prof. Bailey to contain 
microscopic multilocular Foraminifere, the same as found in New Jer- 
sey. Here too, a white indurated clay alternates with C. A little be- 
yond the mouth of the Tchansansan River (or the “ continued Wood 
River’) commences the Coteau de Missouri, along which, on the right, 
is the American desert; along here on both banks occurs this formation. 
D. The last member of the cretaceous group, is a plastic clay, two 
hundred feet thick, divided into unequal beds by clay containing nodules 
of argillaceous iron ; it also contains fragments of limestone, the differ- 
ent varieties of which were indicated for the benefit of future geolo- 
gists. The plastic clay is full of new species of Ammonites, Inocera- 
mus, Belemnites, Baculites, &c. and also remains of vertebrated ani- 
mals which have been described by Dr. Harlan. Dr. Morton has de- 
scribed and figured some of these fossils in the Journal of the Academy 
of Natural Sciences of Philadelphia. Of the few species collected by 
Mr. N., four have been identified with those of the same formation on 
the Atlantic. This formation exhibits the geclogical features of the 
Upper Missouri for four hundred miles, retaining the same lithological 
and fossil character. From specimens and information he thinks this 
formation extends much farther to the west and northwest. 
Nr. N. concluded this portion of his paper by giving an account of 
the topography of the country, and spoke in terms of high commenda- 
tion of the accuracy with which Mr. Catlin had depicted the scenery in 
his interesting volumes. In the midst of the clay banks, and from the 
summit of the hills, dense smoke is frequenly observed to arise from 
crevices in the plastic clay ; he called them pseudo-volcanoes. From 
this fact, and from the occurrence of the light spongy material brought 
down by the waters, and strewed along the shore, many have erro- 
