5 So 



MINERAL VEINS. 



As previously suggested, there appears to be some relation between lodes and 

 the rocks they traverse ; and Mr. Wallace has maintained that the lead-veins 

 of Alston Moor, etc., are due to segregations resulting from the decomposition of 

 the wall-rocks of the veins, and to aqueous deposition from above. There the most 

 important are the Rake-veins, which run east and west, while ' cross-veins ' run 

 north and south ; veins that branch off laterally are known as Flots, and streaks of 

 ore met with in the rocks are known as Pipe-veins.^ 



Iron Ores. 



The occurrence of iron-ore in stratified rocks may be said to be imiversal, for 

 their colouring is largely due to it. Rusty brown tints are due to hydrated 

 peroxide of iron ; brighter red tints, and sometimes darker stains, are due to 

 anhydrous peroxide of iron ; greenish colours are produced by the protosilicate, 

 and bluish tints are imparted by bi-sulphide of iron. (See pp. 275, 285, 408.) 

 Iron-ore has been found in profitable quantities in many formations, but sometimes 

 only in limited areas : it occurs in beds, and nodules, and frequently in veins and 

 pockets. Native iron is found in meteorites, but usually associated with nickel 

 and other substances. 



Among the ores, haematite (anhydrous peroxide or ' red oxide ') occurs in 

 crystalline form as specular iron or iron-glance, in an earthy form as raddle or 

 reddle, and reniform, mammillated, or nodular as kidney-ore. Limonite 

 (hydrous ]3eroxide or ' brown oxide ') occurs in a mammillated or stalactitic form as 

 'brown haematite,' and in an earthy form as ochre and bog iron-ore. (See 

 p. '513.) Gothite is a crystalline form, possessing the same composition. Magne- 

 tite (peroxide and protoxide) possesses magnetic properties, and is known some- 

 times as 'Lodestone.' 



Spathose iron-ore (siderite or chalybite, essentially carbonate of protoxide 

 of iron), is a mineral whose composition is subject to considerable variation, the 

 carbonates of lime, magnesia, and protoxide of manganese frequently replacing to a 

 greater or less extent the carbonate of iron. Moreover, this carbonate of iron 

 is frequently associated with certain argillaceous impurities which interfere with its 

 crystallization, and give rise to the dark-coloured massive varieties called clay-band 

 or clay-ironstones. These impure argillaceous carbonates, — which are so profusely 

 distributed throughout our Coal-measures, partly as regular seams of variable 

 thickness, and partly as nodular concretions, — constitute the ore which, in this 

 country, yields nearly two-thirds of our iron.- (See p. 203.) 



Mr. Bauerman observes that the septaria, or cement-stone nodules, found in the 

 London Clay, may be regarded as clay-ironstones, in which the protoxide of iron is 

 in great part replaced by lime. 



Some clay-ironstones exhibit a concretionary form, called ' cone in cone ' as the 

 seam of ironstone breaks into conical forms with the bases of the cones at top and 

 bottom of the seam, and their apices pointing towards each other. The surfaces 

 of these cones are corrugated by small horizontal fretted wavelets, or ridges, rather 

 resembling those on the outside of some stalactites, and each cone is concentrically 

 enveloped by several coats, the surface of each being similarly corrugated.'* 

 According to Mr. Sorby, this has been produced by concretionary crystallization 

 after the deposition of the rock.* (See p. 177.) 



Spathose iron-ore is worked in the Devonian rocks of the Brendon Hills in 



1 Wallace, The Laws which Regulate the Deposition of Lead Ore in Veins, 

 1861 ; Prestwich, Geology, Chemical and Physical ; see also Robert Were Fox, 

 Proc. G. S. iii. 9. 



2 R. Hunt and F. W. Rudler, Guide to Mus. Tract. Geol. ed. 4, p. 109 ; 

 R. Meade, Coal and Iron Industries of the United Kingdom, 1882. 



3 Manual of Geology, by J. B. Jukes and A. Geikie, 1872, p. 312. 



* Brit. Assoc. 1859 ; see also J. Dickinson, on 'Jackstones' at Merthyr Tydvil. 

 Q. J. ii. 131 ; J. Young, Min. Mag. vi. 13 ; Trans. Geol. Soc. Glasgow, 

 viii. I, 1886; Gresley, G. Mag. 1SS7, p. 17; and Rev. J. Yates on 'Curl,' 

 T. G. S. v. 375. 



