76 ON THE GEOLOGY AND NATURAL HISTORY 
while further reactions—well understood by the chemist—upon the lime contained in the 
clay bed, produce the masses and crystals of selenite that are observed in the lower portion 
of this interesting deposit. This is the theory which, with some little confidence, we have 
formed of these pseudo-volcanoes.” 
About two miles above the mouth of White river, the burnt appearance is again visible 
in this bed. Near this locality there is a seam formed of an accumulation of saline matter, 
a yellow substance like sulphur, selenite, and the hydrated peroxide of iron, three feet in 
thickness, the whole presenting a variety of colors. Large masses have fallen down at 
the base of the bluffs.* 
Formation No. 4 oF GENERAL SECTION. 
This formation is geologically the most important one in the Cretaceous system of the 
Northwest, not only from its thickness and its geographical distribution, but also on account 
of its influence on the agricultural capacities of the country. It is only second in interest 
to the succeeding bed in number, beauty, and variety of its organic remains. Commenc- 
ing about ten miles above the mouth of James river, where it is seen only in thin outliers, 
capping the distant hills and bluffs, it continues, gradually assuming a greater thickness as 
we ascend the Missouri, until we reach the Great Bend, where it monopolizes the whole 
region, giving to the country underlaid by it a most gloomy and sterile aspect. At the 
Great Bend it attains a thickness of two hundred feet, and continues to occupy the coun- 
try bordering the Missouri to the mouth of Grand river, where, in consequence of the 
northwesterly dip of the strata, it passes gradually beneath the water-level of the river. 
The general lithological character of the formation, the boundaries of which have been 
described above, is a dark ash-colored clay, varying, however, in color and structure in 
different localities. From the Great Bend to the mouth of the Shyenne river, it shows 
but little of its shaly character. Its general stratification is horizontal, but the layers are 
broken into numerous irregular fragments. Not unfrequently the layers of clay have a 
seam of gray sand between them. ‘This bed contains great quantities of a whitish saline 
substance,t a yellow material like sulphur, and an abundance of ferruginous matter, which 
* Since this report was written I think I have obtained good evidence that the bituminous bed forms the base 
of No. 4 instead of the top of No.8. In several places near Bijoux hills the surface of the marly portion of No. 3 
exhibits the appearance of erosion to a considerable extent prior to the deposition of the bituminous clays. 
+ This white substance is found disseminated to a greater or less extent throughout all the Cretaceous and Ter- 
tiary beds of the Northwest. It effloresces on the surface of the hills or bluffs, giving them a snow-white appear- 
ance. It impregnates the water that issues from or flows over these beds, rendering it exceedingly disagreeable in 
its taste and purgative in its effects. It sometimes covers an area of considerable extent where the water has dried 
away in the autumn from a depression in the surface. Near the mountains it is sometimes scen covering the 
I s 
