694 DYNAMICAL GEOLOGY. 



limestones, — of which the chalk affords an example, — even a low 

 heat could hardly have- been necessary. The silica was distributed 

 through the calcareous mud of the sea bottom, in the form of Diatoms, 

 Polycystines, and siliceous spicules of Sponges, and therefore was in 

 the soluble state ; and the solution of this silica took place within the 

 mass of the deposit. The tendency of matter of one kind to concrete 

 together led to the forming of flint-nodules and the silicifying of shells 

 and other foreign substances. 



3. Through Oxydation. — The oxydation of the iron of ferriferous 

 minerals, in the destruction of rocks described above, is also a forma- 

 tive process. It usually results, as has been stated, in making the 

 brown hydrous oxyd, limonite, unless either the climate is a dry one, 

 or the temperature is near or above the boiling point, when the red 

 oxyd, hematite, is formed. Further, accumulations of iron ores in 

 great beds have been thus made. Carbonates containing iron and 

 sulphids of iron have been the chief sources of the ore ; but, where 

 these were present to start the process, all other iron-bearing minerals 

 at hand have contributed to the end. 



In a large number of cases, the rock has decomposed and left the 

 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 wei'e 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. 158). The beds made in marshes are in general less pure than 



