THE CONTENTS OF THE SOIL 



33 



uniting lime and sulfuric acid, sulfate of lime or gypsum is pro- 

 duced, is chemical action. 



31a. Knowledge of the work of the earthworm in building 

 soils dates practically from the issue of Darwin's remarkable 

 book, '-The Formation of Vegetable Mould, through the Action of 

 Worms," which the reader should consult for particulars. The 

 subject is also considered briefly in King's "Soil," Chap, i., which 

 also discusses the general means of soil-building. 



32a. As an example of the formation of organic soils in the 

 tropics, read accounts of the mangrove. Its mode of propagation 





Fig. 4. A delta in an orchard. 



is explained, with illustrations, in Bailey's "Lessons with Plants," 

 pp. 371-374 ; the tree is also described in Chap. v. of Gaye's 

 "Great World's Farm." As an example of a formation of a peat 

 bog by the growth of sphagnum, read Ganong "On Raised Peat- 

 bogs in New Brunswick," Botanical Gazette, pp. 123-126, May, 

 1891. Sphagnum is moss which grows in cold bogs. Nurserymen 

 and florists use it in the packing of plants. 



