96 ELEMENTARY SCIENCE 



fertile; this is because of the thorough mixing of materials 

 which occurs in the process of their formation; the result 

 is usually a complex mixture which is very good for the 

 growth of plants. Residual soil whose parent rock was 

 sandstone is sandy and generally infertile. Residual clay 

 comes from shale, and is generally more fertile than very 

 sandy soil, but it is usually heavy and difficult to till. 

 (Tillage means the working of soil for crops, as by plough- 

 ing, harrowing, etc.) Residual soil derived from limestone 

 is usually fertile and tillable; a certain amount of lime in 

 the soil is very desirable for plant growth. 



Transported soils may be subclassified on the basis of 

 the transporting agencies. Thus we have alluvial soils, 

 which are composed of materials (sediment) transported 

 and deposited by rivers. These soils are highly fertile 

 and easily tillable, both on account of their texture and 

 the levelness of their surface. So it is not surprising to 

 find that agriculture and civilization first developed near 

 the mouths and along the lower stretches of great rivers, 

 such as the Nile in Egypt and the Hwang Ho in China. 

 For along these ancient rivers there are great areas of 

 alluvial soil, very excellent for farming, and capable of 

 supporting a dense population. Such rivers, where they 

 flow through level country, develop flood-plains, that 

 is, plains which have been levelled over and built up by 

 the wide floods of the river (see Fig. 42). "Ancient civiliza- 

 tions were confined so generally to rich flood-plain soils that 

 the period before 800 B. C. has been called the Fluvial 

 Period" (Salisbury). In the United States the rich "bot- 

 tom-lands" of river- valleys are famous for their fertility, 

 and are of high value, though subject to the risk of loss 

 by flood of fences, houses, and livestock. 



