io8 THE BOOK OF NATURE STUDY 



The delta of the Nile during inundations is similarly covered 

 with a fine sheet of silt, to which it owes its astonishing fertility, 

 for this silt has been derived by the river from floods which have 

 washed away the finest and best of the soil from higher up stream. 

 But in the dry intervals between the inundations the wind drifts 

 sand in from the desert, and this is also spread out, by the wind, 

 into a thin coat. Thus the deposit is a laminated one, and consists 

 of alternate lamince of silt and sand. As the material dries it 

 tends to split up into thin layers parallel to the lamination. 



Now many fine-grained rocks which make up the earth's crust are 

 found to exhibit this very type of lamination (Fig. 12). They split 

 into thin layers which may be merely planes of parting, as in the 

 silt of an English river, or they may consist of alternating mud and 

 sand like the Nile deposit, or they may be layers differing in some 

 degree in colour, texture, or composition. Moreover, the materials 

 of which such rocks are made are like mud and sand, the only 

 difference being that the laminated rocks are harder and more 

 solid than the laminated sediments. Consequently the pre- 

 sumption arises that these rocks may possibly have been formed 

 by the deposit of similar sediment which has been subsequently 

 consolidated. 



The deposit just described is called alluvial, and it may generally 

 be found wherever a river bed flattens out laterally or longitudin- 

 ally (Fig. 58). Alluvial plains are famous for their fertility, though 

 there is the inconvenience that they are occasionally flooded. After 

 a flood it may happen that the water has taken a new course, but 

 even if it has not done this, the mere fact that a river frequently cuts 

 into one bank more than another will cause it to shift its position on 

 its alluvial plain. Travelling at slow speed it is easily turned aside, 

 and meanders over its plain, at one time at one side or other, and 

 at another time in the middle of its valley. When it enters a 

 lake similar debris is spread over the lake bed, and as the supply 

 of mud to the river fluctuates in amount, depending on the work 

 of rain at higher levels, the deposit on the lake floor will be inter- 

 mittent, and hence it is likely to be laminated. The lake may be 

 ultimately filled up and obliterated in this way, and many lakes are 

 to be seen not yet quite filled up but with deltas of detritus spread- 

 ing out from the mouths of the chief silt-carrying rivers (Fig. 16). 



