THE WORK OF RUXXIXG WATER. 203 



dence. and so of the superposition of one delta on another.^ The delta 

 of the Yukon has a sea margin of 70 miles, and extends more than 

 100 miles inland. The delta of the Rhone has also had a remarkable 

 gro\\1:h. considering the size and the histon' of the stream. Aries, near 

 the debouchure of the stream, was 14 to 16 miles inland in the fourth 

 century B.C.. and is now 30 miles inland.^ The Rhone has also built 

 a great delta in Lake Geneva, and its lower delta is built of sediment 

 gathered below the lake. The Po has built a delta 14 miles beyond 

 Adria. the port which gave its name to the Adriatic Sea. The exten- 

 sion of this delta has been at the average rate of about 50 feet per year, 

 but recently, on accomit of artificial embankments, the rate has been 

 much more rapid.' The Ganges and Brahmaputra together have made 

 a delta of great size. Its area is sometimes estimated to be as high 

 as 50,000 or 60.000 sc^uare miles, and its head is more than 200 miles 

 from the sea.* The head of the Xile delta is 90 miles from the sea, and 

 it has a coastal border of ISO miles. The head of the delta of the Hoang- 

 Ho is about 300 miles from the coast, and its seaward border has a 

 length of about 400 miles, though T^*ith some highland interruptions.* 



After a delta has been built into a lake, the lake may disappear, 

 leaving the delta out of water. Such " fossil " deltas, if so recently 

 exposed that erosion has not destroyed their distinctive features, are 

 readily recognized by their flat tops, their abrupt and lobate fronts, 

 and their characteristic structure. They are often a means of deter- 

 mining the foiiner existence of extinct lakes,* or the former higher 

 levels of lakes which still exist.' Elevated deltas on seashores show 

 either a rise of the land or a depression of the sea-level. 



The material which is carried along the coasts or shores from the 

 mouths of riA'ers may take on various and peculiar forms, according 



* Russell. Rivers of Xorth America, p. 132. 



2 Prestwich. Chemical and Physical Geology. Vol. I. p. So. 



3 Geike. Text -book of Geology. 3d ed., p. 402. 



' Medlicott and Blanford. Geology of India. Chap. XVII: :Medlicott, Records of 

 the Geological Survey of India, ISSl ; Oldham. Geology- of India. 2d ed . Chap. XVII; 

 and Ferguson. Q. J. G. S.. Vol. XIX, pp. 321-54. The extent of this and other deltas 

 is variously stated, probably because it is difficult to determine the exact position 

 of its head and borders. 



5 Dana. Manual of Geology-. 4th ed., p. 19S. 



' Salisbury- and Kiimmel. Lake Passaic. Ann. Rept. of the State Geologist of Xew 

 Jersev. 1S93, and Jour, of Geol., Vol. Ill, p. 533. 



' Gilbert. Lake Bonneville, Mono. I, L". S. GeoL Surv 



