44 SOILS. 



several minerals constituting the common iron ores require 

 mention. One of these is : 



Iron Spar or siderite; carbonate of iron, corresponds in com- 

 position to calcite and dolomite and crystallizes in the same 

 form. It sometimes occurs in large masses and is an import- 

 ant iron ore, brownish-white in color, and when compact re- 

 sists the attack of atmospheric oxygen remarkably well. Like 

 the carbonates of lime and magnesia, it is soluble in carbonated 

 water, and its deposits are undoubtedly formed from such 

 solutions. The latter are copiously formed wherever ferment- 

 ing or decaying organic matter is in contact with iron-bearing 

 materials, such as rust-colored sands or clays ; and if the 

 solution so formed can percolate without coming in contact 

 with air, iron-spar is formed. But whenever the solution 

 comes in contact with air, it absorbs oxygen and the ferrous 

 carbonate is converted into ferric hydrate or rust, mineralogi- 

 cally known as : 



Liuioiiite or brown iron ore. This ore is frequently found 

 deposited on the upper surface of clay layers traversing sandy 

 strata, the clay having arrested the carbonate solution and thus 

 given time to the air to effect the change. Sometimes such 

 deposits form great masses in rock-caves, fissure-veins, or 

 crevices; and like siderite, it is an important iron ore, though 

 frequently quite impure, as in the case of bog ore, which is 

 formed in ill-drained subsoils. It is also sometimes found as 

 the residue from the weathering of rocks rich in hornblende 

 or pyroxene, and in this, as well as in other cases, is pulveru- 

 lent, constituting yelli >:<. ochre. It makes a rust-colored streak 

 on biscuit porcelain or unglazed queensware. // is the coloring 

 material of all yellow or "' red" soils and clays, as well as of 

 brown sandstones, which are cemented by it. 



As is well known, such clays and sandstones become dark 

 red by heating or " burning," as in the case of common brick 

 clays; the brown or yellow ferric hydrate losing its water and 

 becoming red ferric oxid. The latter sometimes occurs in 

 nature in the impure, pulverulent condition, constituting " red 

 ochre " ; but more commonly and abundantly it is found in the 

 form of 



Hematite or red iron ore, which is sometimes formed in 



