19 



$th December, 1882. 



The President, R. L. Patterson, Esq., in the Chair. 



A Paper was read by Joseph J. Murphy, Esq., F.G.S., on 

 DELTAS. 



This paper was illustrated by a map of the Delta of the Nile. 



A Delta was defined as consisting of one or more alluvial 

 islands formed at the mouth of a river. 



Running water is constantly wearing away the surface of the 

 ground, and depositing elsewhere the material so carried away. 

 The former of these actions is known as denudation, the latter as 

 deposition. As the quantity of matter is unchangeable, the total 

 amount of denudation effected by any river with its tributaries 

 must be exactly equalled by the deposition effected by the same. 

 This however is not always visible to the eye, because part of 

 the deposition takes place at the bottom of the sea, beyond the 

 mouth of the river. Denudation preponderates in the upper 

 part of the valley of a river, and deposition in its lower part. 

 In that part of a valley where there is more denudation than 

 deposition, which is the state of things with which we are 

 familiar, the slope of the ground is towards the stream ; but 

 where deposition exceeds denudation, and where consequently 

 the land on each side of the river consists of its own alluvium, the 

 immediate banks of the river are the highest part of the country, 

 and the slope of the ground is away from the river. This is 

 well known to, be the case along the lower Mississippi. The 

 reason obviously is, that silt is deposited during inundations 



