WINDING COURSE OF RIVERS. 23 



of fineness graduate insensibly into one another. But, if we repeat the 

 experiment,, the coarsest material will fall upon the finest of the previ- 

 ous experiment, and then graduate similarly upward. If we examine 

 the deposit thus made, we observe a distinct line of junction between 

 the first and the second deposit. This is stratification, or lamination. 

 For every repetition of the experiment a distinct lamina is formed. It 

 is evident, therefore, that to produce stratification two conditions are 

 necessary, namely : 1. An heterogeneous material ; and, 2. An inter- 

 mittently acting cause. Now, these two conditions are always present 

 in Nature where sediments are depositing. Into every body of still 

 water, as a lake or sea, rivers bring heterogeneous material torn from 

 the land ; but this process is not equable, being increased in the case of 

 small streams by every rain, and in large rivers by the annual floods. 

 Therefore, sedimentary deposits in still water are always stratified. 



In running water the case is somewhat different. If the stream 

 runs with a velocity at all times the same, then with every repetition 

 of the foregoing experiment the same kind of material falls on the 

 same spot — gravel on gravel, sand on sand, and mud on mud — and 

 there will be no stratification. In running water, therefore, another 

 condition is necessary, namely, a variable current. For, when the 

 velocity increases, coarser material will be carried and deposited where 

 finer was previously deposited ; when the velocity decreases, finer will 

 be deposited on coarser, and very perfect stratification is the result. 

 Now, these three conditions are always present in every natural cur- 

 rent. The velocity of every river-current varies not only very greatly 

 in different portions of the year, as in seasons of low water and seasons 

 of flood, but also (from the constant shifting of the subordinate cur- 

 rents of the stream) from day to day, from hour to hour, and even 

 from moment to moment. It follows, therefore, that deposits in run- 

 ning water are also always stratified. Sometimes extreme beauty and 

 distinctness of stratification in the deposits of large rivers are due to 

 the fact that the different branches flood at different seasons, and bring 

 down differently colored sediments. 



We may, therefore, announce it as a law, that all sedimentary de- 

 posits are stratified ; and, conversely, that all stratified masses in 

 whic7i the stratification is the result of sorted material are sedimentary 

 in their origin. Upon this law is founded almost all geological rea- 

 soning. 



4. — Winding Course of Rivers. 



The winding course of rivers is due partly to erosion and partly to 

 sedimentary deposit. It is most conspicuous and most easily studied 

 in rivers which run through extensive alluvial deposit. If the chan- 

 nel of such a river be made perfectly straight by artificial means, very 

 soon some portion of the bank a little softer than the rest will be exca- 



