198 DYNAMICAL GEOLOGY. 



pointj the view of Lyell), through the pressure of the surface deposits on a layer of mud 

 which overlies the Port Hudson clay, or older alluvium of the river. Some carbo-hydro- 

 gen gas is given out, arising from the decomposition of animal or vegetable matters in 

 the mud. The mud-discharges tend to increase the shallowness of the waters and push 

 out the land into the Gulf waters. Mr. Hllgard states, in 1871, that Morgan's mud-lump, 

 in the marsh of Southwest Pass, had been active for 25 years, and during the time the 

 bars had moved gulfward a mile and a half. He closes his paper with a remark (vol. i. 

 435) relating to the distance to which the Southwest Pass must extend in order that there 

 shall be no danger of mud-lumps within the channel. The Eads jetties have since then 

 been made along this pass, in order to give it greater depth. It has secured the depth ; 

 but with danger from this source still existing, as Professor Hilgard has observed. 



According to Humphreys and Abbot, the outer crest of the bar of the Soutliwest 

 Pass, the principal one of the Mississippi, advances into the Gulf 338 feet annually, over a 

 width of 11,500 feet ; and the erosive power is only about Jj of its depositing power. 

 The depth of the Gulf, where the bar is now formed, being 100 feet, the profile and other 

 dimensions of tlie river, in connection with the above-mentioned rate of deposit, give for 

 the difference between the cubical contents of yearly deposit and erosion 255,000,000 

 cubic, feet, or a mass 1 mile square and 9 feet thiclc : tliis, therefore, is the volume 

 of earthy matter pushed into the Gulf each year at the Southwest Pass. The quantities of 

 earthy matter pushed along by the several passes being in proportion to their volumes of 

 discharge, the whole amount thus carried yearly to the Gulf is 750,000,000 cubic feet, 

 or a mass 1 mile square and 27 feet thick. As the cubical contents of the wliole mass 

 of the bar of the Southwest Pass are equal to a solid 1 mile square and 490 feet thick, 

 it would require 55 years to form the bar as it now exists, or, in otlier words, to establish 

 the equilibrium between the advancing rates of erosion and deposit. Hilgard has shown 

 that, about New Orleans, the modern alluvium has a depth of only 31 to 56 feet, there 

 existing below this the alluvial clay, etc., of the Port Hudson group. 



The delta of the Hoang Ho (Yellow River) extends along the coast from near Peking, 

 on the north beyond the Pei Ho, to Hung-tse Lake, on the south, where it joins the 

 plains of the Yang-tse-Kiang. The distance is 400 miles ; but the mountainous coast- 

 province of Shan-Tung is to be excluded. From the coast, the delta extends westward 

 for 300 miles. The river is liere useless for navigation. The whole delta region would be 

 under water during flood seasons except for drainage by artificial dikes and canals of 

 great length ; and these have required constant supervision. At long intervals, the 

 great river has broken loose and swept over the immense area with devastating floods, 

 and ended its mad career with change of cliannel from the river Pei Ho, or some place 

 near it, on the north, to a southeast route ; or the reverse. In 1820 it occupied a southeast 

 channel, emptying into the Yellow Sea, near latitude 33^° N. By 1858 this channel was 

 dry ; and after some years of uncontrolled waters, it took a new channel into the Gulf 

 of Pe-chi-li, 300 miles north. In the autumn of 1887, a new break occurred near Kai 

 Fung, in Ho-Nan ; but the waters instead of resuming the old channel which they left after 

 1852 took a course south from Kai Fung to the Cl^a, 70 miles, and then struck off east- 

 southeastward to the Hoei Ho and the sea. The Chinese have succeeded in leading off tlie 

 upper part of the wandering waters into the old channel mentioned above, leaving the more 

 southern part in its new channel. The first of such changes recorded in Chinese annals 

 occurred in 2293 b.c. ; a second, owing to Chinese care, not until 602 b.c. Several have 

 occurred since. The Mississippi has its disastrous floods, but no chance for such changes. 



(4) Lakes. — The discharge of lakes, like that of rivers, is (1) evapo- 

 rational or upward; (2) gravitational or downward; and (3) surficial,^ sea- 



1 The word superficial is too various in its significations to express the right idea. Surficial 

 is like SM?'/"ace in having for its prefix tlie Frencli abbreviation siu- in place of super. 



