146 LAKE DEPOSITS 



as conditions remain the same, particles of similar size and weight 

 are thrown down at the same spot. If sand, gravel, mud, and 

 clay be shaken in a jar of water and then allowed to stand, the 

 various materials will settle to the bottom in the order of their 

 coarseness, the finest coming down last. Yet, in this case, the 

 change from one kind of material to another will be so gradual, 

 that no well-defined layers will appear, to produce which deposi- 

 tion must cease at intervals, or the kind of material be changed. 

 Each layer represents a time of deposition followed by a pause, 

 which allows the surface particles to assume a somewhat different 

 arrangement from what they otherwise would do. The planes of 

 contact between the successive layers are called the bedding or 

 stratification planes, and it is along these that the mass of sedi- 

 ment most readily parts. The sorting power of the water in 

 a lake thus causes the coarser materials to be thrown down near 

 shore, and the finer to be carried farther out, but changes in the 

 transporting power, caused by storms, high water, and the like, 

 change the place of deposition for particles of a given size, and 

 thus pile gravel on sand, and sand on mud, or vice versa. 



Owing to the way in which the materials are arranged, lake 

 deposits betray the form of the basin in which they were laid 

 down. Around the old shore line are masses of coarse materials, 

 with deltas interspersed, to mark the mouths of streams, while 

 towards the middle of the basin, quantities of fine mud and clay 

 have accumulated. An excellent example of such a deserted lake 

 basin is that known as Lake Bonneville in Utah, of which Salt 

 Lake is the shrunken remnant. The drying up of this lake, which 

 was once fresh and had an outlet northward to the Snake River, 

 is an event geologically so recent, that its form and size, its shores 

 and islands, its high and low stages, in short, its history, can be 

 made out with great clearness, as has been admirably done by 

 Mr. Gilbert of the United States Geological Survey. At its time 

 of greatest extension, Lake Bonneville had an area of 19,750 

 square miles and a maximum depth of 1050 feet, while Salt Lake 

 (which is variable) had in 1869 an area of 2170 miles and an 

 extreme depth of 46 feet. Around the ancient shores are beauti- 



