138 



EFFECTS OF TRANSPORT. 



deposits are for the most part nearly horizontal. Gravel and sand 

 which the waves wash upon coasts, are also deposited at vtiy 

 small angles, and slopes of ten degrees are exceptions, even in 

 localities exposed to the strongest billows; most frequently they 

 are much less, and nearly horizontal. 



35. It frequently happens, during the drift or transportation of 

 matters by currents, and by freshets in rivers, when the bottom is 

 disturbed, that effects analogous to those of sea-winds on dunes 

 are produced. Ridges are formed across the current ; various mat- 

 ters, pushed over these initial hillocks, accumulate behind them, 

 forming a ta'lus of successive fallings, which impart the structure 

 represented in^o-. 217. If the river change its course, the undu- 

 lated surface of the first deposit is soon levelled, and quiet deposits 

 are formed above (Jig- 218), from which the preceding may be 

 distinguished by the particular structure attributable to the circum- 

 stances of its formation. 



Fig. 217. Fig. 218. 



Structure produced by the transportation of materials. 



These effects, resulting from a mixture of rapid and tranquil 

 deposits (that is, deposits formed from rapid currents and tranquil 

 waters), are very clearly seen in alluvions on river banks, and par- 

 ticularly in deltas, which terminate their course when the waters 

 have excavated some ravine near by. We then perceive that the 

 mass of the deposit is formed of horizontal layers, having a surface 

 more or less undulated (Jig. 219), which are distinguished from 







Fig. 219. Structure of alluvions in rivers. 



ach other by the size of the component parts, by the colour, by 

 the structure produced by rapid accumulation, either by pushing 

 forward the matters- in the direction of the ordinary current, as in 

 the deposits a and 6, or in a different direction, as in the deposit c, 

 which indicate counter-currents formed at one time or another. 

 Often there are particular masses, rf, formed here and there, which 

 ordinarily consist of coarser gravel, or of different organic debris. 



35. What effects result from transportation or drift 7 



