I. O. Russell—The Jordan-Arabah and the Dead Sea. 343 
elevations of the horizontal surfaces of barriers and embankments, 
in the case of fossil lakes, furnish the most accurate of all shore 
records for determining former water levels. 
Detras.—A delta formed bya high grade stream, when seen in 
section, presents a well-marked tripartite structure. The middle 
member consists of water-worn gravel, more or less thoroughly 
assorted, in layers sloping lakeward at an angle corresponding with 
its angle of stability in water. The thickness of this deposit in- 
creases as the delta is advanced, and in lakes with precipitous borders 
may become several hundred feet deep near the lakeward margin. 
Beneath the inclined gravels, especially in the case of deltas that 
have been prolonged some distance into a lake, are lacustrine clays 
and marls, which are usually crumpled and otherwise deformed, 
owing to the weight of the mass of material superimposed upon 
them. Above the inclined gravel is a deposit of unassorted or 
irregularly assorted debris, which is thinnest toward the periphery 
of the delta and thickest towards its apex. The history of the 
growth of a delta may be inferred from its structure, and need not 
be described here. 
We have not attempted even a sketch of all the features of lake 
shores that are of assistance to the geologist in determining the history 
of a fossil lake, partially for want of space, but principally for the 
reason that an exhaustive analysis of lake-shore phenomena is 
already accessible to the student.’ 
MecuanicaL SeprMents AND CuemicaL Precrerrares.—The de- 
posits usually formed in lakes, as is well known, are evenly stratified 
clays and marls, which are in many instances charged with shells. 
When lakes are without outlet, however, and become saline owing to 
the concentration of their water by evaporation, their faunas are 
exterminated or greatly modified, and the mineral matter held in 
solution is precipitated when the process has progressed sufficiently. 
If concentration continues without interruption, there will be a 
regular sequence in the precipitates formed, beginning under normal 
conditions, with calcium carbonate, followed by calcium sulphate, 
sodium chloride and other salts. 
Chemical precipitates originating in the manner mentioned above 
may be deposited on the sides of a lake basin with rocks and stones 
for nuclei, as is the case very commonly with incrustations of cal- 
careous tufa; or they may be precipitated generally over the lake 
bottom and become mingled with clays and marls deposited simul- 
taneously. When sedimentation is taking place but slowly, layers 
of various salts may attain a depth of many feet, and should uniform 
conditions continue, may even become hundreds of feet in thickness. 
After the precipitation of saline deposits of this nature a lake might 
expand owing to a climatic change and be freshened either on account 
of the increase in supply, or by the waters rising sufficiently to find 
an outlet, and thus flood out the previously concentrated brine. 
In such instances the beds of calcareous tufa, gypsum and common 
1 “The Topographic Features of Lake Shores,” by G. K. Gilbert, in Fifth 
Annual Report of the U. 8. Geological Survey, 1883-84, Washington, 1885. 
