AQUEOUS AGENCIES. 



35 



mouths of rivers, viz., the river bringing sediment and 

 making land, and the sea carrying it away and destroying 

 land. If the former prevails, a delta is formed ; if the 

 latter, an estuary. 



FIG. 14. Delta of Ganges and Brahmapootra. (From De la Beche.) 



Mode of Formation. We are apt to imagine that we 

 can not observe these phenomena except by becoming 

 travelers. On the contrary, we may observe them in every 

 little stream emptying into a pond. In every such case 

 we shall observe a sand-flat over which the stream runs 

 in many rills, that often change their position. Such a 

 sand-flat is a delta. If we watch the process, we shall see 

 that the stream before entering the pond carries sedi- 

 ment, perhaps is muddy. As soon as it strikes the still 

 water, it spreads out in all directions, the velocity is 

 checked, the sediment falls, the bottom is built up to the 

 surface, and the delta commences. The sand or mud is 

 now carried over the delta, and dumped beyond. Thus 

 the delta grows from day to day (Fig. 15, a). The stream, 



