DELTAS. 29 



to the time, though many thousand years distant, when this lake will 

 be entirely filled up. After leaving the lake the Eh one again gathers 



Fig. 21. 



sediment from tributaries flowing in below the lake, and forms another 

 delta where it empties into the Mediterranean. Many examples of 

 lakelets partially filled, or entirely filled and converted into meadows, 

 are found among the Sierra Mountains. 



In the section view (Fig. 21) we have represented the strata as 

 irregular and highly inclined. This is called oblique lamination. This 

 can only occur when a rapid stream, bearing abundant coarse material, 

 rushes into still water. But, in the case of large rivers flowing long 

 distances and bearing only the finest sediment, the stratification is 

 much more regular and nearly horizontal. ^o^ 



Rate of Growth. — There have been several attempts to estimate the 

 rate of growth of deltas, in order to base thereon an estimate of their 

 age. The delta of the Rhone in Lake Geneva has advanced at least 

 one and a half mile since the occupation of that country by the Romans ; 

 for the ancient town Porta Valesia (now Port Valais), which stood then 

 on the margin of the lake, is now one and a half mile inland. The 

 delta of the same river at its mouth in the Mediterranean is said to 

 have advanced twenty-six kilometres, or sixteen miles, since 400 b. c, 

 or thirteen miles during the Christian era.* The delta of the Po has 

 advanced twenty miles since the time of Augustus ; for the town Adria, 

 a seaport at that time, is now twenty miles inland. But the most elab- 

 orate observations have been made on the Mississippi. This river, as 

 seen in Fig. 19, has pushed its way into the Gulf in a most extraor- 

 dinary manner. According to Thomassy,f and also Humphrey and 

 Abbot, the rate of advance is about one mile in sixteen years. The 

 rate of progress in the deltas mentioned has, however, probably not 

 been uniform. There are special reasons for their more rapid advance 

 at the present time. In the case of the Po, the successful leveeing of 

 this river has transferred to the sea the whole of the sediment which 

 would otherwise have been spread over the flood-plain. In the case of 

 the Mississippi, for many centuries the principal portion of the deposit 

 has been confined to a narrow strip but a few miles wide, and the ad- 

 vance has been proportionately rapid. For this reason the river has 



* Archives des Sciences, vol. li, p. 157. f Geologie pratique de la Louisiane. 



