ON THE EARTH'S HISTORY 



tion of the island of Krakatoa in 1883, the dust which 

 was the product of that mighty explosion was carried 

 round the world, and even in England we saw the dust 

 particles furnishing extraordinary colours in sunset skies. 



In dry countries, especially in the large tracts of 

 Central Asia and of Africa, the air is often so thick 

 with a fine yellow dust that the sun's light struggles 

 through it as through a London fog. The dust settles 

 on everything, and after many centuries a deposit, which 

 may be hundreds of feet deep, is thus accumulated on the 

 surface of the land. Some of the ancient cities of the old 

 world, Nineveh and Babylon for example, after being long 

 abandoned by man, have gradually been buried under the 

 fine soil which the wind blew over them. Even in 

 England the Roman town of Silchester, not far from 

 Reading, after falling into decay when its inhabitants 

 left it, has been buried under the accumulations of two 

 thousand years, and its walls and floors now lie under- 

 ground and have to be carefully unearthed in order to 

 lay them bare. But we need not seek these exceptional 

 cases in order to perceive what the wind is doing with 

 sand and the fine dust of the earth's uppermost layers. 

 At many places round the coast are sand-dunes. On 

 sandy shores, exposed to the winds that blow off the 

 sea, the sand is dried and carried away from the beach, 

 gathering into long mounds or ridges which run parallel 

 to the coast-line. These ridges are often fifty or sixty 

 feet, sometimes even more than 250 feet high, with deep 

 troughs and irregular hollows between them, and they 

 sometimes form a strip several miles broad bordering the 



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