50 Transactions of the 



The sulphate of barium occurs ia small, thin, flat tables, some- 

 times pure white, but more generally reddish, owing to the presence 

 of iron. It is found in the small cavities which abound in the ore, 

 and also in the sandstone matrix. 



The next point to be discussed is, how the ore was deposited as 

 we now find it. It is clear that the stalactites could only have 

 been formed by the action of water containing iron in solution, 

 but this is equally true of the whole mass of iron ore. The ques- 

 tion then arises, whether these beds of haematite were deposited 

 at the same time as the interstratified beds of sandstone and 

 clay, or whether they have been introduced since. The mode of 

 occurrence of haematite deposits in other places, makes it almost 

 certain that the latter is the true hypothesis ; for these deposits 

 are constantly found filling up large cavities or rents which cut 

 the rocks at all angles, and are in no way interstratified as the 

 beds are here. These veins or pockets are especially common in 

 Carboniferous Limestone, and the explanation given in these cases 

 is as follows : — Surface-soil always contains a certain portion of 

 carbon dioxide, due to the oxidation of decaying vegetable matter. 

 Eain falling on the soil takes up some of this carbon dioxide, and 

 the water collecting in streams then has the power of dissolving 

 limestone, forming bicarbonate of calcium. When the water runs 

 for ages between the joints of limestone rocks, large cavities are 

 thus formed — in fact, the latest explanation ofi'ered of the gorge 

 of the Avon is, that it is a great cavern thus formed of which the 

 roof has fallen in. Faults also produce rents in the rocks. Car- 

 bonate of protoxide of iron, FeCOa, is also soluble in water contain- 

 ing carbon dioxide ; but this carbonate is very unstable, like all 

 salts of the protoxide of iron. The iron is very readily oxidised 

 to the state of sesquioxide, Fe^Og; and when solutions containing 

 these sesqui-salts are exposed to heat or pressure, they deposit 

 either basic salts or, if the heat and pressure are sufficiently 

 great, hydrous oxide of iron (limonite), or even anhydrous or red 

 haematite. If, then, water charged with carbonate of the protoxide 

 flows through these cavities, however produced, basic salts or 

 the hsematites will be deposited on the sides, and the basic salts 

 will subsequently yield up their acid and leave hsematite behind. 

 As these rents are often deep under ground, the requisite heat and 

 pressure are easily supplied. The fact that the anhydrous oxide is 

 found in the centre of the lumps may perhaps be thus explained. 

 The effect of the heat alone would have been to convert the whole 

 mass into anhydrous oxide, driving off the whole of the combined 

 water ; but the outside of the lumps being in contact with the 

 porous moist sandstone, retained the water to a certain depth. 

 The further question arises, how the cavities in the sandstone 



