98 RECONNAISSANCE FROM CARROLL, MONTANA, 
itself previously built up, and also to a less extent carrying away the older deposits. It acts 
alone, unaided by any minor tributary streams; for they are dry except in the early season. Even 
during the summer, however, the channel is constantly changing. The mud-and-sand bars which 
are everywhere formed do not long retain their positions, but are moved on down the river and 
heaped up again in other places. Thus the process is one of gradual transferral down the stream ; 
the solid matter going to make one alluvial bank after another until it is finally deposited in the 
Gulf of Mexico. 
It is interesting to note, in this connection, the explanation recently given by Prof. James’ 
Thomson (Proc. Royal Society, 1876) of the origin of the windings of rivers in alluvial plains. He 
shows that, upon hydraulic principles, the velocity of the stream must be greater on the inner bank 
than on the outer, and yet, as shown here, the wearing away takes place upon the outer bank, and 
the deposits are made on the inner bank. This is in part due to the centrifugal force, which tends 
to make the surface-water move away from the inner bank, while its place is taken by a partial 
upward current of the bottom water retarded much by friction. This current moves obliquely 
toward the inner bank, and serves to protect it from the rapid scour of the stream-line. On the 
outer bank, however, there is a tendency of the rapidly-moving surface-water, unimpeded by fric- 
tion, downward against the solid bank; this it tends to wear away, the worn substance is carried 
down to the bottom, where the oblique current spoken of carries it toward the inner bank. Sooner 
or later it will reach this point, and more or less of it will find a resting-place. 
These principles find an application in the flow of the Missouri through its alluvial plain. It 
is on the outer bank of the successive curves of the river that the wear is greatest, and that the 
river has forced its way up to the older bluffs, while on the inner bank the deposits are being made, 
more or less, all the time, sand or mud, or both, according to the relative velocities of the different 
parts of the stream. 
As has been remarked, the work of the river in summer is destructive, and no additions are 
made at this time to the height of the alluvial banks. In spring, the case is very different, and it is 
at that time that the chief deposits of alluvium are made. The river is then full, the snows all over 
the wide area drained by the Missouri are melting, rains are frequent, and a vast amount of material 
is brought in from the surrounding country. The amount of solid matter held in suspension at this 
season is enormous. In floods, the waters rise many feet, overspreading the lower alluvial ground, 
aud in subsiding and evaporating they deposit their load of sand and clay, sometimes covering a well- 
grown and fertile plain with a bed of alluvium afoot and more in thickness. This sometimes takes 
place for a number of successive years at the same points, as is shown by the fact that the roots 
of trees which must have been close to the surface of the ground when they commenced to grow 
were often seen buried beneath from four to six feet of allavium. We could of course only observe 
this on the very edge of the bank, where the water had removed a part of the old alluvium, expos- 
ing to view the roots, and that part of the trunk which 
had been buried. Some of these trees were quite small, 
not more than 3 or 4 inches in diameter, and most of them 
were still living; thus indicating how rapidly such depos- 
its as those referred to are made. The trees were mostly 
cottonwoods and elms, species of rapid growth. That these 
deposits are made very rapidly is also shown by the thick 
Jayers to be noticed in any section of a bank so deposited, 
sometimes a foot or more, perfectly homogeneous. It is 
interesting to note the great variation in the height of the 
perpendicular alluvial banks. From point to point, in some 
cases, it is only three or four feet; in others twenty-five feet 
or more. This depends obviously on the strength of the current, and the extent to which the water 
is backed up. It bears upon the general subject of river-terraces. Not infrequently we observed a 
second terrace above, or rather a long line of high cut bluffs separated from the stream by another 
alluvial plain (see figure 3). This is all of recent origin, and merely means that the river stopped 
washing away the bluffs here, and commenced to fill up at its foot. 
The energy of the stream is at all times directly proportional to the amount of the descending 
