CHARACTERISTICS OF DELTAS 609 



islands may gradually annex these to the land by enclosing them 

 in the growing delta. The hilly province of Shantung has thus 

 been enclosed in the great delta of the Huang-ho, and a number 

 of small islands have been included in the delta of the Aspropota- 

 mos in western Greece. 



The rate of growth of deltas varies greatly and is often con- 

 siderable. Thus the Jaxartes increased by I3>;4 square miles be- 

 tween 1847 s^''*^ 1900. (Andrussow-i '.49.) The delta of the Rhone 

 is said to have lengthened more than 26 kilometers since 400 B. C. 

 The southwest pass of the Mississippi delta grew, according to 

 Captain Talcot, 104 meters in length in 1838, the south pass 85 

 meters, the northeast and southeast passes each 40 meters, and the 

 pass a rOutre 92 meters, giving an average of 80 meters per year 

 for each pass. While this holds for the year in question, it is 

 not possible to consider that such an increase occurs in all years. 

 Indeed, often one year destroys what is built in the preceding year. 

 The Po delta has increased between the years i2(X>-i6oo at an aver- 

 age rate of 25 meters per year, but from 1600 to 1804 its rate of 

 increase was 70 meters per annum. One of the most rapidly grow- 

 ing deltas is that of the Terek, on the Caspian. Within a period of 

 30 years the water has been pushed back 15 kilometers by the 

 growth of the -delta, which increased thus at the rate of half a 

 kilometer per year. The other extreme is shown by the delta of 

 the Danube, which at one of its mouths is not over 4 meters per 

 year, though somewhat more rapid at another. The average in- 

 crease of the Nile is about 4 meters per year, while the delta of 

 the Tiber is estimated to increase at the low rate of i meter per 

 year. According to Pumpelly, the Huang-ho has increased on the 

 average at a rate of 30 meters per year between B. C. 220 and 

 A. D. 1730. 



Thickness or Depth of Deltas. (Credner-14.) The depth of 

 delta deposits on modern sea coasts varies greatly, but is, on the 

 whole, comparatively slight. Thus the mud of the Nile delta is 

 not over 10 or 15 meters thick. It rests on loose sea sand. The 

 delta deposits of the Rhine have a thickness of 60 meters, those of 

 the Rhone over 100 meters. In the Po the depth averages 122 

 meters, though near Venice 172.5 meters were penetrated without 

 reaching bottom. The delta deposits of the Ganges and Brahma- 

 putra rest on older sediments and average only 20 meters in thick- 

 ness. The actual delta deposits of the Mississippi range from 9.5 

 to 16 meters near New Orleans, increasing to 30 meters at the 

 head of the passes, beyond which the thickness rapidly increases. 

 They rest throughout on a stiff blue clay of earlier age. The Rhone 



