TERTIARY AND SECONDARY SOILS. 235 



The sandy varieties of the tertiary soil often consist of al- 

 most pure sand, laying directly upon beds of clay. They 

 may be easily improved by deep ploughing, especially when 

 the clay is not more than 6 or 10 inches below the surface. 

 The sand and clay being mingled together, will improve the 

 texture. The clay often contains carbonate of lime and ox- 

 ide of iron ; two indispensable substances to the fertility of 

 any soil. But as most of the tertiary soils resemble those 

 from other formations, they will be described under the head 

 o^ clayey soils. 



The tertiary soil is of limited extent in New England. It 

 is confined mostly to the region of plastic clay. All the com- 

 mon clay-beds, and the soils resulting, are assigned by Prof. 

 Hitchcock to the diluvium. 



IV. Secondary soils or soils from the secondary rocks. 

 The secondary formation includes a great variety of rock, and 

 consequently a similar variety of soil. It would be useless 

 here to point out all these varieties, as the chemical mode of 

 classification will bring many of them together, as identical 

 in composition, and in their agricultural relations. 



1. The cretaceous or chcdky soil is rarely found in this 

 country, but is very abundant in England. (1) It consists of 

 calcareous earth in the form of chalk or marl, mingled with 

 flint, pebbles or concretions, and will be described under the 

 head calcareous soil. When this soil covers chalk rocks it 

 is white, and reflects the heat, hence it is often cold ; but 

 many varieties of it are very fertile. 



(2) A second variety of the cretaceous soil consists partly 

 of green sand, resembling chlorite or green earth mingled 

 with sand. The green sand often contains large quantities of 

 potash, and has been used in New Jersey as a manure with 

 the most salutary effects. But this variety of soil, in this 

 country, with the exception of New Jersey, does not gene- 

 rally contain potash. 



(3) A third variety consists of blue marl clay, carbonate 



