340 PRACTICAL LESSONS IN SCIENCE. 



some other re-agent. In this way limestones, and feldspathic 

 rocks, carbonates and silicates of calcium, potassium, and sodium , 

 are disintegrated, the calcium and alkalies, becoming carbonates, 

 are removed by solution and the rocks crumble down into sand 

 and clay. 



In this connection may be mentioned the action of freezing 

 water. In northern latitudes the soils and rocks become satu- 

 rated with moisture during the autumn. During winter the water 

 in all the cracks, joints and interstices freezes, and by its expan- 

 sion increases all these spaces, and so on from year to year till 

 as the ice melts in the spring some of the outer particles becom- 

 ing loosened are washed away ; thus soils and rocks are broken 

 up and pulverized in some cases with great rapidity. The work 

 of water is somewhat increased by the mechanical action of fall- 

 ing rain. 



The general effect of air and water aided by heat, frost, grav- 

 ity, and vegetable acids on rocks is called weathering. This ac- 

 tion varies with different rocks and in different climates ; by this 

 process nature transforms rocks into soil and carves out many 

 strange and interesting forms on the rocky surfaces of the earth. 



Soil is formed in various ways, but on level or gently sloping 

 surfaces, much soil is formed by the decomposition of rock in situ 

 by the process of weathering. Perhaps most of the work is done 

 by carbonated water, but other agencies have more or less effect. 



Nearly all rocks are more or less jointed or fissured, so that 

 water charged with carbon dioxide from the air or decaying veg- 

 etation, readily finds its way deep into the rocky strata, reappear- 

 ing at lower levels as springs. During its underground journey the 

 water dissolves more or less material from the rocks over which 

 it passes, so that the water of springs is seldom pure water. 

 Sometimes the water may have passed over insoluble rocks, and 

 issue from the earth as pure, soft water. Or it may have passed 

 over limestone rocks, and loaded with calcium carbonate form a 

 calcareous spring, supplying " hard " water. And according to 

 the nature of the rocks through which the water flows, a spring 

 will be ferruginous, sulphur, saline, alkaline, etc. 



