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CHAPTER XIX. 



ON THE REPOSITORIES OF METALLIC ORES. 



Metallic Matter disseminated through Rocks. — Masses of Metallic Ore. — Metallic 

 Beds. — Metallic Veins. — Rake Veins. — Flat Veins. — Accumulated Veins.— 

 Cross Courses. — The remarkable Structure of the Botallack Mine worked un- 

 der the Sea. — On the Formation of Metallic Ores. — Remarkable Phenomena in 

 Mines. — Stream Works. — Gold disseminated in the Sands of Rivers in Africa, 

 and North and South America.— Rocks in which certain Metallic Ores are found. 



The rocks and strata, described in the preceding chapters, are 

 composed of earthy minerals, sometimes combined with a portion of 

 metallic matter, which in such instances may be regarded as a con- 

 stituent part of rocks. The mineral substances to be described in 

 the present chapter, as forming beds or veins, or irregular masses, or 

 grains imbedded in other rocks, consist of metallic matter either 

 pure, or in combination with sulphur, oxygen, or acids. 



The difference of external character between a pure metal and an 

 fiarth is so great, that we find some difficulty, at first, in conceiving 

 how metallic matter can form beds interstraiified with earthy rocks ; 

 but the discoveries of modern chemistry have shown, that metallic 

 and earthy minerals are closely allied. Nothing can appear more 

 essentially different than a piece of polished iron and a piece of mar- 

 ble or slate; yet if iron be exposed to the action of air and water 

 it is converted into rust, and in this state is known as ochre ; and be- 

 tween ochre and powdered stone there is little difference of external 

 character ; nor would any one, unacquainted with chemistry, suspect 

 that ochre was a metallic mineral. The ochre can, however, be ea- 

 sily reconverted into metallic iron : but, to convert the earths into a 

 metallic substance is a difficult process, — yet, it has been effected ; 

 and it is further proved, that both earths and alkalies are metallic 

 substances combined with oxygen. The metallic nature of the earths 

 being ascertained, we can no longer be surprised that metallic min- 

 erals should be found, intermixed with earthy minerals, in rocks. 

 Iron is found combined with earths in almost all rocks that are not 

 white ; and to the presence of iron they generally owe their colour, 

 whether red, brown, or black. 



The other metals rarely occur, chemically combined with rocks or 

 strata, but are found either disseminated in grains or irregular pieces, 

 or forming beds betw^een earthy strata, or filling veins that intersect 

 rocks in diflerent directions, as represented Plate IV. fig. 4. a and b. 



The metals, except gold and platina, are rarely found pure, but 

 are generally combined, either with sulphur, oxygen, or acids ; in 

 this state, they are called ores. When the metals occur pure, they 

 are called native metals : thus we have native gold, native iron, &c. 



