1 66 SIZE AND GROWTH OF DELTAS. 



the Arve and the Rhone flow far without mixing, the Nahe takes one 

 side of the Rhine, and even in the mining districts of England, the 

 discoloured streams from the different valleys can often be distin- 

 guished along considerable lengths of a united river. 



We shall not further extend our remarks on this subject than by 

 mentioning a few instances of the actual extent of the deltas of great 

 rivers. The whole area of the dry delta of the Po and the Adige, 

 and other rivers which contribute to extend the same line of 

 coast, must exceed 2000 square miles, and within the last 2000 years 

 a space of 100 miles in length, and from 2 to 20 miles in breadth, 

 has been added to the land. The area of the Nilotic delta is about 

 12,000 miles, and according to Girard the surface of Upper Egypt has 

 been raised by the Nile sediment 6 feet 4 inches since the Christian era ; 

 the area of the Rhone delta is 1500 square miles ; of the Niger delta 

 is 25,000 square miles. 1 The delta of the Ganges, without reckoning 

 that of the Burarnpootra, which has now become conterminous, is 

 considerably more than double that of the Nile, and its head com- 

 mences at a distance of 220 miles in a direct line from the sea. The 

 base of this magnificent delta is 200 miles in length. 2 



The fen lands of Lincolnshire, Huntingdonshire, and Cambridge- 

 shire occupy 1000 square miles, and the levels in connection with the 

 Humber 300 or 400 square miles. 



It has been attempted to deduce the age of existing continents from 

 the rate of increase of the deltas of rivers within the historic era. 

 Thus the Nile was supposed by Herodotus to have formed Lower 

 Egypt ; and he states that if diverted into the Red Sea, it would fill 

 that gulf with its deposits in less than 20,000, or even 10,000 years. 

 Since the time of Herodotus it is supposed that the increase on the 

 Nilotic delta has been, upon an average, one mile and a quarter. The 

 average annual growth of the delta of the Po, opposite Adria, which 

 was once on the edge of the Adriatic and is now thirteen miles inland, 

 was, from 1200 to 1600 A.D., 25 metres, and from 1600 to 1800, 70 

 metres ; a very rapid increase of rate, probably connected with the 

 increasing shallowness of the sea. 



But all inferences from observations of this nature, and similar 

 ones on the shallowing and conversion into land of |he upper ends of 

 lakes, can only lead to speculative results, without a knowledge 

 of the original depth of the sea or lake at all points over which 

 the river sediment has flowed, a datum very difficult to obtain ; 

 for it is not by the area of the delta, but by the cubic content of the 

 sediment transported that the time occupied in the process is to be 

 ascertained. 



Rate of Sub-^lrial Denudation. The average rate at which sub- 

 ferial denudation, however, goes on, is now determined with some 

 approximation to accuracy. Professor Geikie finds that a large river 

 carries away from its basin a mass of sediment which, if uniformly 

 spread, would amount to ^Vs f a f* ever y y ear ? so that taking the 



Dr. Fitton, " Geology of Hastings." 2 Lyell. 



