192 AIR AND LIFE. 



by storms, and are thus enabled to obtain a foothold upon islands or con- 

 tinents to which their own forces could not have brought them. And 

 micro-organisms, last but by no means least, as far as importance, not 

 size, is concerned, make great use of atmospheric movements. They 

 possess no means of locomotion, and have no limbs to carry them to a 

 distance; but the wind makes good this deficiency, and takes good 

 care to scatter them far and wide. There are many epidemics prop- 

 agated from city to city, from country to country; there are death- 

 working as well as beneficent microbes scattered over the whole face 

 of the earth, and thus air is again an agent of death and of life 

 by its contents, no less than by its essential constituents. But air 

 contains also a large proportion of non-living matter, of dead dust 

 as contrasted with the living dust just referred to. Such dust also 

 is scattered far and wide, and there is no doubt but that it may be 

 carried from China to North America, and from the New World to 

 Europe or Africa. This dispersal of dust may be of some importance 

 in agriculture; at all events it plays no insignificant part- in geology, 

 and all have heard of the influence of winds in the formation and 

 migration of dunes on the seacoasts. This influence is often important. 

 In France, in the region at the south of Bordeaux and in certain parts 

 of Brittany, the wind has brought so much sand from the dry shores 

 at low tide that man has been compelled to retreat and to desert his 

 villages. These, gradually covered by the particles carried by the 

 winds, have finally been entirely engulfed and buried, as were Hercu- 

 laueum and Pompeii of old under the cinders of Vesuvius, and the only 

 remnant of a once inhabited and prosperous hamlet is a spire which 

 sticks out of the plain of sand. Analogous phenomena are to be 

 observed in all countries. In 1889, according to Mr. George P. Mer- 

 rill, a storm occurred in Dakota during which the soil was torn up 

 to the depth of 4 or 5 inches, and the particles accumulated easily in 

 recognizable sand dunes. In the western plains of North America, 

 also, the same event occurs, and these dunes, when Once formed, travel 

 and migrate from place to place. Some miles north of Lake Winne- 

 mucca (Nevada), Mr. Bussell found a series of such dunes, 40 miles 

 long, 8 miles wide, some of which were 75 feet high. Near Alkali Lake 

 other dunes are 200 and 300 feet high, and on the eastern shore of Lake 

 Michigan simila; 1 dunes have advanced upon forests, which they have 

 invaded, smothered, and destroyed, and the tops only of the trees, dead 

 as a matter of course, emerge above the hillocks of sand. 



It should be observed that particles of sand driven by the wind exert 

 an erosive influence. They act as files, and gradually wear away the 

 rocks which they unceasingly batter, and thus the wind works in two 

 ways toward the leveling of the globe; indirectly cooperating .with 

 water and with frost it helps to disintegrate the elements of rocks; 

 and when they are broken down it carries the particles, which are the 

 ultimate result of such disintegration, to the plains and to the sea. 



