710 DYNAMICAL GEOLOGY. 



bed of iron ore — mostly limonite — in its place. This is the fact in 

 the region of Lower Silurian schists of the Green Mountains, as first 

 explained by Percival, and of their continuation in New Jersey, Penn- 

 sylvania, Virginia, Tennessee, Georgia and Alabama. The lamina- 

 tion of the schist may be sometimes detected in the ore bed, when its 

 minerals have disappeared. In one of the mines of Richmond, Mass. 

 (the Leete ore -bed), it is apparent that the source of the iron was 

 mainly a ferriferous carbonate. A high limestone ledge stands just 

 along-side of the mine, to the north ; and, within the deep and large 

 excavation, in the midst of the ore, there are some few beds of very 

 compact gray carbonate of iron still remaining, which are conformable 

 or nearly so in dip with those of the limestone ledge a hundred yards 

 off. The rock from which the limonite originated was probably, there- 

 fore, this carbonate ; possibly, portions of it that were less compact or 

 more permeable to moisture. 



The iron of exposed rocks undergoing decomposition is very com- 

 monly washed out of them into low places or marshes, and there de- 

 posited, making beds of cellular limonite, called " bog iron ore." Such 

 beds often contain nuts and leaves, petrified by the oxyd of iron. The 

 iron, when carried by the waters, is in solution as bicarbonate, or com- 

 bined with organic acids derived from the soil. The change to limonite 

 takes place where the waters have a chance to stand and evaporate. 

 In this way, vast beds of ore have been made, even those of Archaean 

 time (p. 153). The beds made in marshes are in general less pure than 

 those formed in place, because a marsh gathers much dead animal matter, 

 and therefore the ore usually contains phosphates (p. 59). Even much 

 of the Archaean ore contains phosphate of lime (apatite) in visible grains. 



The oxydation of iron has also taken place without any attending 

 destruction of rocks. In the Marquette iron region, and others, there 

 are imbedded octahedrons of iron ore, which are now hematite Fe 2 3 , 

 or, what is the same, FeOi, but which were originally magnetite, FeOI, 

 as is proved by their having the crystalline form of magnetite, instead 

 of that of hematite. They show that the great bed of ore, of which 

 they are a part, has been in some way oxydized (receiving in it a sixth 

 more of oxygen). This was probably done through the aid of the 

 moisture penetrating the whole, when at a high temperature. Igneous 

 rocks usually contain magnetite rather than hematite. 



Consolidation of rocks is another effect, in some cases, of the pro- 

 duction of iron ore. Limonite becomes distributed among pebbles, and 

 thereby makes an ironstone conglomerate. 



The waters, filtering through soil and gravel, often take up enough oxyd of iron to 

 cement a bed of pebbles tying, at a lower level, on another layer sufficiently close in tex- 

 ture to hold the water and give the iron a chance to deposit; and this is one way in 

 which what is called hard-pan is sometimes made. The underlying impervious bed is 



