TO YELLOWSTONE NATIONAL PARK. 99 
water; aud hence is immensely greater in spring than in summer. This energy is probably all 
expended in overcoming friction, and in carrying the load of solid matter. The difference in the 
amount of detritus held in suspension by the stream in early July and in late September was very 
marked—at the latter time the stream seemed to have to a great extent cleared itself. This is 
doubtless due to the diminished volume of the water, in consequence of which the carrying power 
of the stream was so much diminished. <A river of this character scems to act as a destructive 
agent rather through the weight and moving force of its own water than by means of the abrading 
power of the solid matter it carries with it. 
A true upper terrace was not observed at any point above the mouth of the Yellowstone. At 
points below, it was not uncommon to see one hugging the lignite bluffs, and separated from the 
river by a wide alluvial plain. Whether it be a true terrace or only a recent deposit is doubtful. 
Such a place was noticed a short distance below Fort Buford, where the water must once have 
spread over an immense area, pointing to the time when the Missouri was a much larger stream 
than it is at present. 
Above Carroll, the river-bottom becomes much less wide, and, although sometimes flowing 
through valleys more or less broad, the stream generally passes along between and close beneath 
frowning banks of washed clays and sands. The undermining of the banks takes place here in 
the same manner as where they are alluvial; but, owing to the greater hardness and thickness 
of the ofder rocks upon which the water acts, the process is much more slow. It goes on con- 
stantly, however, so that at last a great mass of the rock above, perhaps a hundred feet in thick- 
ness, deprived of its support, slips down into the water. This has occurred at many points, and 
gives to the rocks, as viewed from the river, a great variety of dip, which has been considered by 
some observers as indicating an extensive disturbance of these beds, due to the elevation of the 
mountain-ranges of this section of the country. We cannot doubt, however, that all these apparent 
disturbances are purely local, and have been caused by the action of running water. 
FROM CARROLL TO BOX ELDER CREEK. 
Fort Pierre Group of the Cretaceous. 
The clay bluffs at Carroll rise abruptly above the alluvial bottom on both sides of the river. 
They belong to what Dr. Hayden has called the Fort Pierre Group, Cretaceous No. 4. These bluffs 
consist of a dark-blue to purplish-black laminated clay, occasionally stained with iron, and some- 
times containing very thin layers of white sand. They are remarkably constant in character from 
top to bottom. Dr. Hayden has stated in general that the clays of No. 4 are not laminated ; 
but this is not true of those which came under our observation. 
The characteristic features of this clay are (1) the large calcareous concretions, which will 
be spoken of more particularly in connection with Crooked Creek; (2) the plates and crystals of 
transparent gypsum, or'selenite ; and (3) the alkaline deposits. 
The selenite plates are quite conspicuous, as they lie on the surface of the ground, and glisten 
brilliantly as the sun strikes them. In general, they are irregular crystalline fragments; but occa- 
sionally perfect crystals are found of the form common in the clay of Poland, Ohio. The surface 
of all these fragments is roughened and etched by the solvent action of the water which has 
flowed over them. ‘These etchings are most distinct on the clinopinacoid, and are similar to those 
described by Baumhauer as having been produced artificially by the action of caustic potash. The 
