284 



GENERAL SCIENCE 



is subject to quick changes in temperature, the weathering 

 takes place much more rapidly than in a warm and dry 

 climate (Figure 244). 



Experiment 72. Fill a small bottle with water, cork tightly, 

 and place in a freezing mixture. What is the effect upon the 

 bottle? What would be the effect if water in the pores or cracks 

 of a rock should freeze ? 



Some rocks weather much more rapidly than others. 

 It is interesting to note the changes that take place on 



the tombstones due to 

 weathering. In the east- 

 ern half of the United 

 States many of the in- 

 scriptions are illegible on 

 tombstones that have been 

 standing for 50 or 60 

 years. 



The uneven contraction 

 and expansion of rocks 

 due to the summer heat 

 sometimes causes them to 

 chip and crack. The roots of plants penetrate the crevices 

 of rocks and split them apart, as they grow in size. Water 

 at times may acquire acid properties in passing through 

 decaying vegetable material, which makes it a solvent for 

 some of the materials of the rock. 



How Soil Is Made. If we examine the soil in any field, 

 we find that there is a dark layer of humus on top (Figure 

 245). The richness of the soil may be quite accurately 

 estimated from the depth of this layer of dark earth. 

 The deeper the layer of dark earth the richer the soil. 

 The dark color of this layer is caused by the decayed plants 

 it contains. Plants turn black as they rot. As we go 



FIG. 244. Exposed Shales, Showing 

 Effects of Weathering. 



