LESSONS IN GEOLOGY. 339 



follows the leader as waves follow each other. Where there is 

 abundance of moisture not much sand can be moved during the 

 year, and vegetation soon begins to grow on the dunes, and may 

 in time so cover them as to stop the action of the wind and the 

 dune becomes permanent. Sometimes these dunes interfere with 

 drainage forming lakes, ponds, and marshes, as alongthe eastern 

 shore of Lake Michigan and western coast of France. Dunes 

 sometimes serve as barriers against the sea and sometimes as 

 wind brakes. This action of the wind in moving sand can be 

 studied almost anywhere on windy days, even in the street or 

 schoolhouse yard. 



Of the rain and snow that falls on the earth some is soon 

 evaporated, some is taken up by the soil, some by plants, some 

 sinks into the ground to reappear as springs, and some from 

 25 per cent, to 30 per cent. flows into the ponds, brooks and riv- 

 ers. Water acts on the soils and rocks chemically and mechanic- 

 ally. Water, when it reaches the soil, contains a little air and 

 carbon dioxide, dust, living germs, etc., so that one effect is to 

 fertilize the soil as well as to moisten it. Pure water usually pro- 

 tects the rock it covers, but water in conjunction with air is an 

 active oxidizing agent. 



A thin pellicle of the oxide is formed on the surface, which if not 

 washed away thickens from the inside until a thick crust covers 

 the rock. The oxidation of iron and iron compounds is the most 

 interesting and important. Sulphide of iron is widely dissemi- 

 nated through nearly all kinds of rock, and under the influence of 

 air and water both the sulphur and iron are oxidized, sometimes 

 forming ferrous sulphate; and sometimes other bases take the 

 acid, leaving the iron free to become the ferric oxide. Thus 

 many kinds of rocks are entirely broken up, some parts passing 

 away in solution, allowing the whole to disintegrate rapidly. 

 Carbonate of iron is also acted on in much the same way, the iron 

 changing to the ferric oxide, allowing the rocks containing it to 

 decay rapidly. 



Water dissolves salt, sodium chloride, without aid, but in 

 most cases of solution water is aided by carbon dioxide or 



