ATMOSPHERE. 629 



foik, England, between Hunstanton and Weybourne, the sand-hills 

 are fifty to sixty feet high. 



2. Additions to land by means of drift-sands. — The drift-sand hills are 

 a means of recovering lands from the sea. The appearance of a 

 bank at the water's edge off an estuary at the mouth of a stream is 

 followed by the formation of a beach, and then the raising of the 

 hills of sand by the winds, which enlarge till they sometimes close 

 up the estuary, exclude the tides, and thus aid in the recovery of 

 the land by the depositions of the river-detritus. Lyell observes 

 that at Yarmouth, England, thousands of acres of cultivated land 

 have thus been gained from a former estuary. In all such results 

 the action of the waves in first forming the beach is a very import- 

 ant part of the whole. 



3. Destructive effects of drift-sands. — Dunes. — Dunes are regions of 

 loose drift-sand near the sea. In Norfolk, England, between Hun- 

 stanton and Weybourne, the drift-sands have travelled inland 

 with great destructive effects, burying farms and houses. They 

 reach, however, but a few miles from the coast-line, and were it not 

 that the sea-shore itself is being undermined by the waves, and is 

 thus moving landward, the effects would soon reach their limit. 



In the desert latitudes, drift-sands are more extended in their 

 effects. 



4. Dust-showers.-— Sands are sometimes taken up by whirlwinds or 

 in heavy gales into the higher regions of the atmosphere and 

 transported to great distances. 



In 1812, volcanic ashes were carried from the island of St. Vincent 

 to Barbadoes, 60 to 70 miles ; and in 1835, from the volcano of Co- 

 seguina in Guatemala to Jamaica, 800 miles. 



Showers of grayish and reddish dust sometimes fall on vessels in 

 the Atlantic off the African coast, and over southern Europe ; and 

 when they come down with rain they produce "blood-rains." 

 Ehrenberg has found that the dust of these showers is to a great 

 extent made up of microscopic organisms.* The figures on the 

 adjoining page represent the species from a single shower which 

 came down about Lyons on October 17, 1846. The amount which 

 fell at the time was estimated by Ehrenberg at 720,000 lbs. ; and 

 about one-eighth consisted of these organisms, making 90,000 lbs. 

 of them. 



The species figured by Ehrenberg include thirty-nine species of siliceous Di- 

 atoms (figs. 1-65) ; twenty -five of what he calls Phytolitharia, only a few of 



* See his work entitled " Passat-staub und Blut-regen," 4to, 1847, and Amer. 

 Jour. Sci. [2] xi. 372. 



