IRON 



929 



Spathic ores vary very much in appearance according to the. amount of manganesQ 

 present ; the richest, such as those of Siegen, are coarsely crystalline with curved 

 cleavage faces of a pearly lustre; while the Styrian, which contains but little man- 

 ganese, although otherwise very pure, is of a dull granular fracture. The Cornish 

 ore from Perran is also very dull and unlike the crystalline mineral. 



Coal pleasure iron stones. The compact carbonate of iron has no relation ex- 

 ternally with the sparry variety. It comprehends most of the clay-ironstones, parti- 

 cularly those which occur in flattened spheroidal masses of various sizes among the 

 coal-measures. The colour of this ore is often a yellowish brown, reddish grey, or a 

 dirty brick-red. Its fracture is close grained, it is easily scratched, and gives a yellow- 

 ish brown or grey powder. It adheres to the tongue, has an odour slightly argillaceous 

 when breathed upon ; blackens at the blowpipe without melting, and becomes attract- 

 able by the magnet after calcination. The ironstones of the coal-formation admit of 

 a natural division into two great classes, viz. the argillaceous, and the blackband or 

 carbonaceous. The earthy or lithoid carbonates occur in some regions in the upper 

 limestone shales, and they extend upwards through the coal-measures proper towards 

 their higher limits ; they likewise occur in extensive beds in the Jurassic formation, 

 particularly in North Yorkshire ; near the upper limit of the Lias, or base of the 

 oolites proper ; and again higher, as nodules and perhaps as beds, in the middle oolites, 

 or Oxford clays. They are also found extensively as courses of nodules in the 

 Wealden series, and as beds in the Greensand. When these grey carbonates contain 

 lime in abundance, and when clay is not largely present, they are sometimes changed 

 by atmospheric influences into hydrated haematites ; in Northamptonshire, for example, 

 and widely in France. The only great coal-fields in Great Britain in which these 

 ores do not occur in suflicient abundance to form the basis of a large production of 

 iron, are those of Northumberland and Durham, and of Lancashire. The great im- 

 portance of the argillaceous and blackband ironstones of our coal-fields is clearly 

 shown by the fact, that they supply a very large proportion of the entire iron produced 

 (Blackwell). They vary considerably in their percentage of iron, which is generally 

 not more than 30 to 33 per cent., but occasionally ranges as high as 40 per cent. 

 They are rarely used when they contain less than 25 per cent. The varying pro- 

 portions of iron, silica, and alumina which they contain is shown in the subjoined 

 analyses of the ore from different localities: 



