244 ROCKS AND SOIL 



gritty, had sharp edges, and I was afraid it would injure the seeds 

 so I discarded that. There was some " leaf mold," as the man 

 there called it, at the very top under some shrubs. He recom- 

 mended that, but I didn't want the mold on my seeds. The black 

 earth extending down a foot from the top looked too dirty so I 

 discarded that. There was a streak of gravel, but I knew tiny 

 roots couldn't penetrate the pebbles. Then I saw two more kinds 

 of soil. One was a yellowish sand that looked good to me, but 

 our experiment showed it was porous and so when I watered the 

 plants the water would run through. The last was a bed of clay. 

 I could see that this was fine grained. It was in such lumps I 

 could hardly get it out. I could easily see that if I watered it that 

 the water would have hard work to get out, so I chose the clay. 

 I also knew that it was richer in food value for plants than sand is. 



THE REVIEW SUMMARY 



In preparing a summary of what you have learned in this unit, 

 you will want to place emphasis on the big ideas which have 

 come out of the applications of the facts you have learned and the 

 demonstrations you have seen. These big ideas we call general- 

 izations. For this unit they are as follows : 



1. The surface of the earth is being constantly changed by the 

 forces of water, wind, heat, cold, and other agents. 



2. These changes are always going on night and day, winter 

 and summer. 



3. We can recognize whether these changes are recent or very 

 old by the appearance of the earth's surface. 



4. We know that different forms of life once inhabited the 

 earth because of remains in the rocks called fossils. 



5. Soil has been formed and now is being formed by the weather- 

 ing and erosion of rocks. 



6. Plants use the elements of the soil in order to live. 



Before making your review summary, test your knowledge of 

 the facts of the unit by checking over the text so as to be sure you 

 know the facts underlying the generalizations ; then, using the 

 generalization, the material in the text, and in addition everything 

 you have read, seen, or done yourself, make a summary outline 

 for your workbook. This outline you may use when you make 

 a recitation. 



