OF THE UPPER MISSOURI. 111 
of rocks. ‘This exceedingly curious feature in the country was often pointed out to me as 
a mystery by the voyagers as we ascended and descended the river. The same pheno 
menon occurs on Pratt’s creek, about fifteen miles below Bijoux hills, though on a much 
smaller scale, consisting of a series of local belts of land either entirely destitute or wholly 
covered with angular or slightly worn rocks. We have, first, a belt 15 or 20 yards wide, 
covered very thickly with erratic blocks; second, an interval of 150 yards almost free from 
rocks; third, a belt of rocks similar to the first, and so continues for several miles. ‘These 
alternate belts occupy a large area in this region, having a northwest and southeast range, 
and show conclusively the source from whence these rocks were derived as well as the 
agency which transported them from their parent bed. Bijoux hills, which are from 500 
to 700 feet high above the bed of the Missouri, are covered with “ erratics,” many of which 
are masses of limestone containing fossils, as Trilobites, Crinoids, Corals, and Brachiopoda 
of palxozoic types. The above facts show clearly the great interest which surrounds this 
subject, and that it is well worthy of a careful investigation. 
4th. Bottom Prairie. 
The broad fertile bottom prairies of the Missouri are included in the above division of 
the superficial deposits, and constitute a most interesting geological feature. These bottom 
prairies form by far the most fertile lands of the West, the vegetable soil sometimes ex- 
tending downwards to the depth of twenty or thirty feet, and covered with tall sedge grass 
and flowering plants, with here and there a grove of gigantic cottonwood trees. These 
bottom lands are quite extensive along the Missouri from the mouth of the Kansas river 
to Council bluffs, but attain their greatest width between Council bluffs and Sioux city. 
On the Iowa side of the river the bottom seems to be continuous for one hundred miles, 
varying in width from five to thirty miles. The Vermilion prairie commences at the 
mouth of the Big Sioux and extends with very little interruption to Dorion’s hills, a dis- 
tance of sixty miles, and varies in width from one to eight or ten miles. The materials 
which compose the Bottom Prairies seem to have been derived to a great extent from the 
calcareous and silicious marls of the Tertiary, mingled with the clays of the Cretaceous 
strata of the Upper Missouri, and the surface is covered with a thick vegetable mould, from 
the annual decay of an enormous growth of vegetation. 
5th. Alluvium. 
The distinction between Alluvium and Bottom Prairie is very gsm “a es yee 
Missouri river. It is quite evident that the latter, as restricted in the previous division of 
the Quaternary deposit, could not have been formed by any agencies in operation at the 
