no THE BOOK OF NATURE STUDY 



truck load, are spread out parallel to the surface at the point 

 where they are tipped down. In addition to the oblique lamina- 

 tion, it will be seen that the masses of pebbles and sand considered 

 as a whole are wedge shaped, not bounded by parallel surfaces, 

 thinning out in one or more and probably in all directions (Fig. 17). 

 This phenomenon is called false-bedding. 



As such structures are produced by the dropping of material 

 which has been rolled along, they are naturally the result of deposit 

 in moving water under the action of currents in shallow depths, 

 and they will be characteristic of coarse-grained deposits. Similar 

 structures are to be found in the rocks of the earth's crust, chiefly 

 among conglomerates and sandstones which yield other evidence 

 of having been formed under precisely these conditions (Fig. 25). 



The deposit of river-borne material on land is only temporary ; 

 the bulk of it is taken up again and eventually carried out to sea. 

 Here it is largely deposited in the form of deltas consisting of 

 fine-grained debris laid on the sea-bed until it is built up to the 

 water level and can be taken possession of by vegetation. Here 

 the lamination structures will originate as a result of the alterna- 

 tion in supply of material, each lamina representing the supply 

 brought down during a definite interval of time, a spate, a rainy 

 season, or a period of rapid disintegration. The deltas at the 

 mouths of the Nile and Ganges cover thousands of square miles, 

 and represent the deposit of matter denuded from large drainage 

 basins. 



Even this does not account for the whole denuded material, 

 for much of it, especially in tidal waters and those swept by 

 marine currents, is carried out to sea and eventually dropped over 

 a large area of the sea bed. The sea margin is generally edged 

 with coarse deposits of shingle and sand, which acquire the usual 

 characters of shallow-water deposits. But the finest material is 

 carried much farther out and spread over many thousands of 

 miles of sea bed. As, however, the water gets more and more 

 still below the surface, it is rare to find mechanically denuded 

 material more than 300 miles from the shore, and it does not 

 usually spread more than 200 miles. Dispersed over so wide an 

 area, the material carried out during any particular interval of 

 time must be spread very thinly over the sea bed. Hence the 



