IRON-ORE OF THE COAL-MEASURES. 383 



Iron- Ore of the Coal- Measures. 



We have already stated that the Coal-measures consist of alternat- 

 ing layers of sandstones, shales, and limestones, containing seams of 

 coal and bands of iron-ore. We have already discussed the mode of 

 occurrence, the varieties, and the theory of accumulation of the coal. 

 We come now to discuss the same points in regard to the iron-ore. 



Mode of Occurrence. — The mode of occurrence of iron-ore is, in 

 many respects, like that of coal. Like coal, it is found in seams, which 

 vary in thickness from a fraction of an inch to forty or fifty feet. 

 Like coal, these very thick seams are apt to be impure, being largely 

 mixed with clay. Seams pure enough to work profitably are seldom 

 more than three or four feet thick. Like coal, the seams are repeated 

 many times in the same section (Fig. 450, p. 347), but without any dis- 

 coverable order of succession. Like coal, the seam is usually underlaid 

 by clay. 



Kinds of Ore, — The form of iron-ore found in all strata, except those 

 containing coal, is usually ferric oxide, either hydrated (brown hema- 

 tite — limonite), or anhydrous (red hematite), or else magnetic oxide ; 

 but in the Coal-measures of this period, and in the Coal-measures of 

 every other period — i. e., in all strata containing coal — the iron is in 

 the form of ferrous carbonate. This is usually mixed with clay, and 

 therefore called clay iron-stone. It is often nodular and mammillated, 

 and called kidney iron-ore. Sometimes it is mixed intimately with car- 

 bonaceous matter, and is called black-band ore. This last very valuable 

 ore is found in Pennsylvania, Ohio, and in Scotland. 



The importance of the association of coal and iron in the same 

 strata can not be overestimated. For this reason, the raising of coal 

 and the manufacture of iron are conducted in connection with each 

 other, and the smelting-f urnaces are often situated at the mouths of 

 the coal-mines. It is easy to understand, therefore, why Great Britain, 

 the greatest coal-producing country in the world, should be also the 

 greatest iron-producing country. Nearly all the iron-ore worked in 

 Great Britain is taken from her coal-measures. In this country, much 

 iron is made from the iron carbonates of the coal-measures, but much 

 also from the peroxide and magnetic ores found elsewhere, especially 

 in Laurentian strata (p. 286). 



The following table gives a comparative view of the annual iron- 

 production, in tons, of the principal iron-producing countries of the 

 world. It will be seen that Great Britain makes more than a third of 

 the iron of the world. The rapid increase in the production of this 

 great agent of civilization is also seen. In 1888 the iron and steel pro- 

 duction of the United States reached the enormous amount of 12,000,- 

 000 tons: 



