112 ON THE GEOLOGY AND NATURAL IIISTORY 
present day, while the former is subject to change from year to year and even from day to 
day. ‘The channel of the Missouri is continually changing from side to side, wearing away 
and removing alluvium, bottom prairie, or any other formation with which the current 
may come in contact. ‘The materials are held in suspension in the waters and are carried 
down the stream to form sandbars, islands, &c. Most of the islands which so thickly dot 
the bed of the Missouri and much of the timber portions along the side of the river are 
formed of alluvium. When the main current of the water changes from one side of the 
bed to the other a sandbar is formed immediately, a thick growth of willows springs up, 
which is soon succeeded by cottonwood, which may become a fine forest before the current 
brings its eroding power again to bear upon it. Near the mouth of the Platte there is an 
island, called Pilot island, formed about fifteen years ago by the wreck of a steamer on a 
sandbar. ‘The loose materials commenced accumulating upon the upper side of the broken 
ship, and at the present time the island covers an area of several acres with a thick 
growth of cottonwood trees from twelve to twenty inches in diameter. The Missouri, 
as well as all its tributaries, furnish countless examples of this deposit, but the land thus 
formed is so subject to removal that it is of comparatively little economical value to the 
settler farther than to furnish a moderate supply of timber. The disadvantages arising 
from their want of permanence are too well known to be mentioned in this connection. 
I have thus presented a mere outline of my notes on the superficial deposits of the 
Northwest, with the view of developing the subject more fully in my future investigations. 
Surface geology has not yet received that attention which its importance demands, and it 
seems quite obvious that a clear understanding of the forces now in operation will be found 
to be of the highest importance in explaining the geological phenomena of the past. 
3 River Terraces. 
So well marked a feature of the valley of the Missouri and its tributaries as the river 
terraces should not be passed over in this connection without a brief notice. I will not 
at this time attempt to account for their existence, but content myself with stating a few 
of my observations. 
All along the valley of the Missouri river, from mouth to source, these terraces are 
more or less conspicuous, and they are also seen in the valley of every important tribu- 
tary. These phenomena are not, however, confined to the Northwest alone, but are ob- 
served to a greater or less extent all over the western portion of ‘the continent, and such 
is their similarity of character, that it points to some uniform cause for their existence. 
In Prof. Dana’s great work forming one of the Reports of the Exploring Expedition series, 
the subject is treated very fully from observations made by the author along the western 
