THE ORIGIN AND DESCENT OF ROCKS. 



425 



Iron Ore-beds. — In a somewhat different way iron ore-deposits are 

 formed by the precipitation of iron oxide or iron carbonate from solutions 

 of ferrous compounds. The ferrous compounds in solution were leached 

 from iron-bearing rocks by percolating waters. The most famihar case 

 is that of iron-bearing springs. On exposure to the air, the iron com- 

 pounds in solution undergo change, and ferric oxide is thrown down, 

 usually forming limonite (Fe203,3H20), but sometimes hematite (Fefi^). 

 This change is common in marshes and gives origin to ' ' bog-ore." Similar 

 deposits take place in certain shallow lakes, and henoe are known as 

 '^lake ore.'' Iron ore sometimes also forms at the bottom of a peaty 

 bed or in muddy soil. In connection with the great coal formations, 

 beds of iron carbonate (siderite) occur. Organic matter seems to play 

 a great part both in the original solution and the later deposition of 

 these ores. From certain soils and clay-beds on which the ancient 

 coal-producing forests grew, the iron has been almost completely removed, 

 either by the action of the roots, or more probably by organic acids 

 arising from their decay and from the decaying vegetation on the surface. 

 On flowing into shallow bodies of water or into marshes, the waters con- 

 taining such dissolved iron compounds usually throw down their iron 

 content either as a carbonate (siderite), or as a hydrous ferric oxide 



(Hmonite). The siderite is formed where de- 

 caying vegetation is present to furnish abun- 

 dant carbon dioxide and to partially protect 

 the iron solution from oxidation, and the 

 hmonite where free oxidation takes place. 

 Sand, silt, clay, or calcium carbonate often ac- 

 cumulates with the iron precipitate, and the 

 result is an impure deposit which becomes 

 an ironstone. Such deposits often become 

 segregated into nodules, as will be explained 

 later. It is thought that diatoms sometimes 

 aid in the deposit of iron ore in shallow waters. 

 Silicious deposits. — In the decomposition 

 of igneous rocks, a certain portion of the sihca, as well as of the bases, 

 is dissolved and carried away in solution. Certain organisms extract 

 this from solution for their skeletons, just as others extract calcium 

 carbonate. The accumulation of these silicious skeletons often forms 

 siHcious rocks. The diatom , radiolarian, and other oozes (Fig. 353) 



Fig. 353. — Diatom ooze. 

 Magnified 150 times. 

 (Murray.) 



