proceedings: geological society 299 



ticularly irregular subsidence on land and the upheavals off shore 

 known as mud lumps, suggest that the Mississippi delta is affected by 

 internal flowage, and in the hope of obtaining important side lights on 

 this and other problems a brief examination has recently been made of 

 many large and small deltas of Europe, northern Africa, and western 

 Asia. The progress made in understanding the Mississippi delta was, 

 however, through the observation of dissimilar, rather than similar 

 characters, for each delta seems to be built on individual specifications. 

 The Mississippi delta appears to be unique in the rapidity of its growth, 

 in the fineness and arrangement of the materials composing it, and in 

 other characters which seem related to these, such as the extreme de- 

 velopment of the bird foot form, the narrowness, depth, and stability 

 of its stream channels, and a condition of unstable equilibrium. 



The stability of a delta appears to depend principally on its thickness, 

 the slope of its front, the proportion of sand in the river's sediment, the 

 extent to which this sediment is washed by the sea or lake, the rate of 

 growth, and the degree to which the fine watery sediment is separated 

 into layers of wide extent. In all characters favoring instability the 

 Mississippi delta seems to lead. The thickness is thousands of feet. 

 The front, though not steeper than that of many sand deltas, is very 

 steep for a mud delta, and might be steeper were it not for flowage. 

 The river delivers to the sea relatively little sand, and most of that is 

 brought at times of high water. The sea does not sort or carry away 

 much of the sediment, so that the delta grows rapidly and layers of 

 sandy silt corresponding to high stages of the river are separated by 

 layers consisting of soft flocculated clay and other fine watery material. 

 All these factors conduce to fluidity. 



The fishes of the Lahontan drainage system of Nevada and their rela- 

 tion to the geology of the region: John Otterbein Snyder. A consider- 

 able part of Nevada and the eastern slopes of the Sierra Nevada of 

 California are drained bj^ rivers, the waters of which do not reach the 

 ocean but ultimately find their way into large lakes where the inflow is 

 balanced by evaporation. The waters of some of these lakes are so 

 charged with mineral salts as to be practically lifeless, while others, 

 Pyramid and Winnemucca lakes, for example, fairly teem with fishes. 

 The more recent geological history of this region has been worked out 

 and narrated in a masterly way by Israel Cook Russell.' His account 

 begins with the Quaternary times, when Lake Lahontan covered a large 

 area \vith a maxim.um depth of more than 880 feet, and continues 

 down to the present, when nothing is left of the ancient lake but its 

 dc-^iccated remains scattered here and there over a rock-bound waste of 

 desert sands. Two of Russell's conclusions are of particular interest 

 when viewed in connection with the results of an investigation of the 

 fish famia of the system: First, that the lake had no outlet; and. sec- 



' Russell, Israel Cook. Geological History of Lake Lahontan, a Quaternary 

 Lake of Northwestern Nevada: U. S. Geol. Surv. Monograph No. 11, Washington. 

 188.5. 



