THE MAJOR SOIL-FORMING MINERALS. 45 



nature by limonite losing its water, but more commonly in 

 different ways. It is but rarely found in soils and is of no 

 special interest in that connection. 



A fourth form of iron ore, quite common in the soils of some 

 regions, is 



Magnetite or magnetic iron ore, also known as lodestone. 

 This mineral, the oxygen-compound of iron corresponding to 

 " blacksmith's scale," also occurs in large masses and is an 

 important and usually a very pure iron ore. It occurs very 

 commonly disseminated through certain rocks, and in their 

 weathering it remains unattacked and thus passes unchanged 

 into the soils and sands, constituting the " black sand " so well 

 known to gold miners and almost universally present in the 

 alluvial soils of the Pacific coast. These black grains are of 

 course attracted by the magnet and can thus be easily recog- 

 nized and extracted. In soils they are simply inert, like quartz 

 sand. 



But while the ore is of little interest to the farmer, it is quite 

 otherwise with the compound of this oxid with water, the 

 ferroso-ferric hydrate; intermediate in composition between 

 the white ferrous and the brown ferric hydrates. As men- 

 tioned above, the black silicate minerals, such as hornblende 

 and pyroxene, are bottle-green when seen in thin sections. 

 Nearly the same color, with modifications running toward blue 

 and bright green, is often seen in natural clays and rocks, 

 and is almost always caused by the ferroso-ferric hydrate. 

 Such materials always become red or reddish when heated by 

 the formation of red ferric oxid ; while when exposed to damp 

 air, they assume the rust color of ferric hydrate. 



Reduction of ferric hydrate in ill-drained soils. When such 

 oxidized, rust-colored clays or soils are exposed to the action 

 of fermenting organic matter, the first effect observed is the 

 change of color from rusty to bluish or greenish, by the reduc- 

 tion of the ferric to ferroso-ferric hydrate. Afterward, if the 

 action is continued, the solution of ferrous carbonate (see 

 above) may be formed, and the greenish or bluish color may 

 disappear. 



The importance of this reaction to farming practice lies in 

 the fact that the blue or green tint, wherever it occurs, indi- 

 cates a lack of aeration, usually by the stagnation of water, in 



