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112 HEPOHT — 1859. 



" In beds of limestone existing further to the east of those in which the just now 

 mentioned fossil bones occurred, and which are evidently a continuation of the 

 same series of rocks, little or no dolomite is included ; they are also particularly 

 free from caverns and generally from stalactitic deposits, presenting us with similar 

 limestone rocks, for the most part unaltered by those changes which produce the 

 phenomena of dolomization and caverns. These rocks are coloured black by the 

 oxides of iron and manganese, and are traversed by numerous white calcareous 

 veins ; they form a part of the black marble so frequently employed for statuary 

 urposes in this part of England. Distinct bluish-black slate and argillaceous 

 ydraulic limestone beds are of very frequent occurrence in them, the beds contain- 

 ing occasionally iron pyrites, which, by the action of the weather, tinge the sur- 

 faces of the argillaceous and calcareous rocks of a rusty yellow colour.^ Applying 

 now the above facts to account for the alteration of our cavern-containing rocks, 

 we may legitimately suppose that their previously contained pyrites might by its 

 decomposition yield a supply of sulphuric acid and sulphate of iron, and that these 

 compounds, reacting upon the limestone in their neighbourhood, would (in presence 

 of the air) finally produce sulphate of lime and peroxide of iron — the disengaged 

 carbonic acid at the same time generated, affording the required means for effecting 

 (in presence of moisture) the decomposition of its slaty layers ; these in their thus 

 disintegrated condition being afterwards compressed by means of superposed beds 

 of limestone into a compact series of beds identical with those of pur quarry, and 

 coloured purple in their slaty seams by the above-mentioned peroxides of iron and 

 manganese, — the bicarbonates of magnesia (and also the bicarbonates of iron and 

 manganese) required to produce dolomization being at the same time formed by the 

 action of the carbonic acid upon the masses of limestone, which is found on ana- 

 lysis to contain a sufficiently notable proportion of the necessary ingredients. 



"But the physical evidence that these limestone beds are truly rocks of the black 

 marble series, altered by chemical changes in them, allied to those now pointed out, 

 does not alone rest on the similarity of their strata, allowance being made for the 

 effects of such changes ; the hollow cavities of the black marble are occasionally 

 lined with acute scalene dodecahedrons of calcareous spar, and in the supposed 

 altered series of rocks similar crystals are met with, these being generally corroded 

 on their surface, and thus affording an evidence of a change in the conditions ex- 

 isting after their formation. In connexion with the deposits of stalactite, and in 

 numerous small cavities in the dolomite, other crystals of calc-spar are not unfre- 

 quent ; but under both these circumstances they exhibit different forms, those of the 

 stalactite being generally acute rhombohedrons, whilst the dolomitic cavities are 

 lined with crystals having the figure of obtuse rhombohedrons, combined occasion- 

 ally with the planes of a second rhombohedron, which is more acute. There are, 

 moreover, in these altered strata, instances of the formation of a second crop of 

 crystals in the cavities still occupied by the acutely scalenohedral forms ; and in 

 all the cases I have had an opportunity of observing, these secondary crystals in- 

 variably contain obtusely rhombohedral surfaces. I may also add that there may 

 be considered to be good evidence that the causes connected with the original for- 

 mation of dolomite took place under conditions very different from those existing 

 at the present day ; for not only does the iron pyrites belong to a very persistent 

 variety of that mineral (no marcasite being mixed with it), but the oxide finally 

 seen to result from its decomposition is not a yellow-brown hydrate, like that of 

 the present day, but a red anhydrous peroxide, which would not have been likely 

 unless the temperature at the time was somewhat elevated. 



" During the progress of the study of these rocks, I was able to obtain physical 

 evidence of the presence of all the chemical compounds before described as occur- 

 ring in them, sulphate of lime alone excepted ; this, it may be remembered, I sup- 

 posed to have been removed by the agency of water ; and that means adequate for 

 the removal of this somewhat soluble salt existed, was amply proved by the very 

 numerous caverns produced by the decomposition of the dolomite to which so fre- 

 quent reference has been made. In the lower strata of the quarry the workmen 

 arrived at two very large openings of this kind, in the immediate neighbourhood 

 of the bone cavern, and that these communicated with a plentiful supply of water 

 was easily proved by the splashing sound heard when stones were thrown into 

 them. 



