256 The Reahn of Nature CHAP. 



cut off by a geologically recent elevation of the land. Yet its 

 salinity is less than 2 per cent, while that of the sea averages 

 3. 5. This is because the shelving shores, and particularly the 

 wide shallow inlet of Kara-Baghas, act as natural salt-pans, 

 evaporating the thin layer of water covering them and 

 causing a deposit of crystalline salt, which is thus being 

 gradually withdrawn from solution, while the evaporation is 

 made good by a continual supply of fresh river-water. On 

 account .of the excess of evaporation the surface of the 

 Caspian is now about 90 feet below sea-level, and its shores 

 form a sunk plain. The Jordan Valley ^ in an equally rain- 

 less region, is a still more remarkable instance of a sunk 

 plain. The Sea of Galilee is a small lake 600 feet below 

 sea-level, and from it the Jordan flows for 100 miles along 

 the line of a great fault in a valley averaging 7 miles in 

 width, and enters the Dead Sea at a level 1290 feet below 

 that of the Mediterranean. 



336. Ice Action. The snow-fields lying on the high 

 parts of mountain ranges above the snow-line ( 163) con- 

 tinually increase by the condensation of vapour from the 

 atmosphere. The weight of the accumulating mass of 

 snow compresses the lower layers, squeezing out the air, 

 and forming compact ice, which, although one of the most 

 brittle substances to a blow, is plastic when subjected to 

 steady pressure. Glaciers or streams of ice flowing down 

 the slopes prevent an excessive accumulation of snow 

 on high mountains. The cause of the plasticity of ice 

 under pressure is usually considered, following Professor 

 J. Thomson's theory, to be that pressure lowers the melting- 

 point ( 72), allowing the lower layer of the mass to liquefy 

 and adapt itself to the surface it rests on ; the relief of 

 pressure thus afforded allows the water to solidify again, 

 and the process is repeated continually. But since ice at 

 very low temperatures is plastic, though in a less degree, 

 the theory of melting by pressure is not a sufficient explana- 

 tion. Messrs. M'Connel and Kidd have recently made 

 experiments which show that while crystals of ice are 

 individually rigid and brittle, a mass of them frozen together 

 is plastic even at low temperatures, the crystals apparently 



