82 GEOLOGY OF SOIL. 



Lime generally exists in the state of plaster, or bone-dust, 

 or sometimes as limestone. In truly calcareous or limy soil, 

 lime may form several per cents. But such soils are the 

 exception, not the rule, of the earth's covering. It is soil, 

 in its universal features, to which attention is here directed. 

 Lime has been very often, perhaps generally, separated from 

 its earthy combinations, and has been separately stated as 

 carbonate of lime in soil; as such it has been too often a 

 product, not an educt of analysis. 



So, too, of potash and soda, when these exist in other 

 forms than as common salts, they are usually in soil in com- 

 bination with its vegetable matter, and are to be considered 

 with that element. 



These considerations authorize the inclusion of clay, iron 

 and magnesia in the insoluble division of soil, while the 

 bone-dust and plaster naturally existing and soluble by the 

 aid of natural water, or more easily by rain water, are to be 

 included in the soluble portion. 



Bearing this division in mind, let the examination of soil 

 be extended to other districts of our country, where the soil 

 resting on an immense field of limestone, underlaid by the 

 rocks which were referred to (16) as fossil iferous, has been 

 examined by the process applied to Massachusetts soils — yet 

 with such modification as to procure a portion of clay, iron 

 and magnesia separately from the insoluble substances. 



Fifteen soils, from Wisconsin and Iowa, gave per 100 

 parts — 



Insoluble in weak acid, ..... 82.500 



Clay, iron, magnesia, 5.600 



Adding these, the sum is . , . . . 88.100 

 Lime in various forms of limestone and plaster, 1.860 

 The soil of these immense regions shows not so intimate 



