THE ATMOSPHERE. 633 



The drifting of dust by winds is remarkably exemplified also in the 

 records of the transportation of volcanic ashes. In 1812, such ashes 

 were carried from St. Vincent to Barbadoes, 60 to 70 miles ; and in 

 1835, from the volcano of Coseguina in Guatemala to Jamaica, a dis- 

 tance of 800 miles. The ocean's bed must have received in past times 

 very large contributions from its many volcanic islands. 



2. Encroachments of Drift-sand ; Danes. — Dunes are regions of 

 loose and moving drift-sand near the sea. They often travel inland, 

 through the agency of the winds, and thus encroach on cultivated 

 fields, forests, and villages. Prof. Winchell states that on Lake Mich- 

 igan the sands are continually shifting with the winds; and at Grand 

 Haven and Sleeping Bear, the forest has become submerged, and 

 " presents the singular spectacle of withered tree-tops projecting a few 

 feet above a waste of sands." The land at this place is extending lake- 

 ward, through the wear and contributions of the arenaceous shore 

 rocks. In Norfolk, England, the drift-sands have buried farms and 

 houses. They reach but a few miles from the coast line ; but here the 

 coast-line is moving landward through eroding waves. By such means, 

 not only bones, shells, tree-trunks, become the fossils of sand heaps, 

 but, in the existing age, even man, his houses, temples, and cities. 



3. Transportation of Living Species, or their Relics. — A tornado 

 that becomes a " water-spout " over a large river or lake, carrying up 

 at its centre great quantities of water, will take up also the ova and 

 smaller life of the waters, and transfer them to other places, and may 

 thus contribute new species to distant lakes or rivers. Land birds and 

 insects are sometimes drifted far out to sea, and so reach oceanic isl- 

 ands, and sometimes in the case of birds another continent. Seeds of 

 many kinds go with the winds. A spider of the ballooning kind, Sar- 

 otes venatorius, has probably traveled around the globe, according to 

 H. C. McCook, crossing oceans and continents, and thus has gained a 

 world-wide distribution. A related species is reported by Darwin as 

 suddenly appearing on the rigging of the Beagle sixty miles from the 

 land. 



Showers of grayish and reddish dust sometimes fall on vessels in 

 the Atlantic off the African coast, and over southern Europe (produ- 

 cing, when they come down with rain, " blood-rains"), the particles of 

 which, as first shown by Ehrenberg, are largely microscopic organisms. 

 The figures on the adjoining page represent the species from a single 

 shower, near Lyons, on October 17, 1846. The whole amount which 

 fell was estimated by Ehrenberg at 720,000 lbs. ; and of this one 

 eighth, or 90,000 lbs., consisted of these organisms. 



The species figured by Ehrenberg (Passnt-staub und Blut-regen, 4to, 1847, and Amer. 

 J. Sci., II. xi. 372), include thirty-nine species of siliceous Diatoms (Figs. 1-65); 



