10 SOIL PHYSICS AND MANAGEMENT 



3. Metamorphic rocks include those that have been changed 

 from their original condition by both physical and chemical agen- 

 cies. These may have been of either aqueous or igneous origin. 

 Changes have given rise to marbles from limestones, slates from 

 shales, and gneisses and schists from igneous rocks. 



QUESTIONS 



1. Define soil. 



2. Which of the elements essential for crops are taken from the air and 



which from the soil? 



3. What is the significance of the hardness of a mineral in the formation of 



soils? 



4. Why should quartz be such a common constituent of soils? 



5. Why is little feldspar found in soils when it is so abundant in rocks? 



6. What is the chief value of zeolites? 



7. What is the importance of non-silicates as soil formers? 



8. Which are the principal non-silicate minerals in soils of humid regions? 



9. How are igneous rocks formed? 



10. Give distinctions between granites, syenites, diorites, and diabases. 



11. Distinguish between chemical precipitates and sedimentary rocks. 



12. How are soils formed from calcareous rocks? 



13. Where do we find soils formed from chalk? From limestone? 



REFERENCES 



1 Clark, F. W., Bulletin 616 U. S. Geological Survey, The Data of Geo- 

 chemistry, 1916, p. 34. 

 Merrill, G. P., Rocks, Rock- Weathering and Soils, 1906, p. 15. 



