6oo THE POPULAR SCIENCE MONTHLY. 



At the deepest point, fresh erosion or corrosion is taking place, while 

 the steep bank adjacent is being rapidly worn away (see Diagram 

 No. III). 



The features to be described can only be satisfactorily observed in 

 time of low water. The bank above the river on the deep side is then 

 generally very high, often rising perpendicularly twenty feet or more 

 above the surface of the water. This high bank, thus exposed to the 

 view of the navigator in the river, affords a most excellent opportu- 

 nity of studying the manner in which the material composing the gen- 

 eral valley has been deposited, the various agencies that combined to 

 form the deposit, and the approximate time required for the accumula- 

 tion of a given thickness of this alluvium. 



These walls of loose earth are always very conspicuously stratified, 

 the layers having various thicknesses and different colors. As many 

 as a dozen distinct strata can usually be seen, often very definitely 

 marked off from one another. The color of these layers enables the 

 observer to determine, with considerable certainty in any case, whether 

 it was due to a wash from the neighboring hills, whose color can be 

 directly compared, or to a deposit from the river itself, brought in 

 time of flood from points higher up, or, as is often the case, from vege- 

 table mold which long immunity from disturbance has allowed to 

 accumulate. Some idea of the time occupied in the total deposit may 

 be formed from the presence of forests of Cottonwood {PojucIks moni- 

 lifera, Ait.) which line the river. These trees are sometimes of great 

 size, measuring three or four feet in diameter, and, although the Cot- 

 tonwood is a rapidly growing tree, there can be no doubt that many 

 of the trees are two or three hundred years old. But the mere pres- 

 ence of these forests standing upon the surface of the latest stratum 

 of the general valley is by no means the only time-measure we have. 

 A careful observer, though merely walking among them, might per- 

 ceive that some of them have their bases buried to some little depth 

 with alluvial earth or vegetable mold. This fact, which would escape 

 any one who was not specially looking for evidences of it, becomes 

 striking when the edges of the strata are viewed from the river. 



As the river wears away the previously formed deposits of its 

 valley, it at length approaches the portion that has had time to become 

 covered with these forests. Undaunted, it attacks this portion also, 

 and begins the work of felling the trees. Their roots are laid bare, 

 the solid earth on which they have stood for ages is swept away, and 

 one after another these ancient giants succumb to the rapacity of the 

 waters, and fall powerless into the raging current. Every step in the 

 process by which this result is accomplished may be seen by watching 

 these eroded banks while floating down the stream. The river, as it 

 passes one of these doomed forests, is choked with snags, through 

 which the surging waters roar, and among which it is extremely diffi- 

 cult and often dangerous to guide a boat. These snags are of all 



