METALLIC MATTER IN ROCKS. METALLIC BEDS. 285 



Metallic ores and native metals are sometimes disseminated in 

 grains through rocks ; and when they are abundant, the whole mass 

 of the rock is worked as a mine ; but this is seldom the case. Tin- 

 stone, or the oxide of tin, is sometimes disseminated, in grains, in 

 granitic rocks in Cornwall, but it is generally in the vicinity of a vein 

 of tin ore, that disseminated grains of tinstone are found in the rock. 

 At Weal Duchy mine, near Callington, silver ore is obtained, both 

 from a vein which intersects the hill, and from the rock itself, at a 

 considerable distance from the vein. From a section of the mine 

 shown me by the proprietor, it appears that in the rock, which is 

 white kilias (a silvery clay slate), the ore is disseminated in various 

 parts, or is collected in bunches. The silver is found native in fila- 

 ments, or in the state of vitreous silver ore, black silver, and ruby 

 silver. Gold frequently occurs in grains, disseminated through solid 

 rocks, or in the sands of rivers. Considerable masses of metallic 

 ore are sometimes found in rocks, particularly of iron ore ; but these 

 masses are generally formed by the meeting of numerous veins, or 

 are parts of metallic beds that are greatly enlarged : — they will be 

 described with beds and veins. 



Metallic Beds. — Some metallic ores occur, taking the form of 

 regular strata in the secondary rocks, or of beds in transition and 

 primary rocks. Ironstone in thin strata, alternates with coal, coal- 

 shale, and sandstone, and has been described with the coal strata, 

 in Chap. VIII. 



Iron ore often forms beds of considerable thickness, interposed be- 

 tween rocks of gneiss, mica-slate, and slate. Metallic ores, in beds 

 or strata, may be regarded as constituent parts of the rocks in which 

 they occur, and must be cotemporaneous with them ; the metallic 

 and the earthy minerals have been deposited at the same time, and 

 have probably, been separated by chemical affinity, during the pro- 

 cess of consolidation. Sometimes, the metallic matter is intermixed 

 with a bed of slate, or of other rocks, in such abundance, that the 

 whole bed is worked as a metallic ore. When a bed of metallic 

 matter swells out, irregularly, to a considerable thickness, it forms 

 masses of ore, which, in some instances, attain the magnitude of 

 small mountains ; — such are the mountains of iron ore in Sweden 

 and Norway. Metallic beds are, however, of limited extent ; they 

 seldom traverse a whole mountain or mountain range, but they grad- 

 ually or suddenly become narrow and terminate, or in the miners' 

 language wedge out. There are few known beds of metallic ores 

 in England ; the principal repositories of metallic matter are in veins. 

 I have however ascertained, that the copper mines formerly wrought 

 in the transition rocks of Cumberland, were beds of copper pyrites, 

 interposed between the beds of the mountains in which they were 

 found, and not intersecting them like veins. The beds of rock be- 

 ing highly inclined, the thin metallic beds between them have been 

 mistaken for veins. I believe that several metallic repositories in 



