IIODGE 
: — OSSIFKKOUS CAVERNS AT OHESTON. 
34-5 
qtirnt oocurrcnco in tliciii, tlie beds conlaiiihig occasionally iron jiyrilcs, which, 
by the action of the weather, tinge the surfaces of the argillaceous and cal- 
caieous rocks of a rusty yellow colour. Applying now the above facts to 
account for the alteration of our cavern-coutaming rocks, we may legitimately 
suppose that their previously contained pyrites might by its decomposition 
yield a supply of sulphuric acid and sulphate of iion, and that these com- 
pounds, reacting upon the limestone in then- neighbourhood, would (iu presence 
of the air) finally produce sulphate of lime and peroxide of iron, the disengaged 
carbonic acid at tlie same time generated aH'ording the rccpiircd means for 
effecting (in presence of moisture) tlie decomposition of its slaty layers ; these 
in their thus disintegrated condition being afterwards conipressed by means of 
superposed beds of limestone into a compact series of beds identical with those 
of our quarry, and coloured purple iu their slaty seams by tlie above-mentioned 
peroxides of iron and manganese. The biearbouates of magnesia, and also the 
bicarbonatcs 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 analysis to contain a sufB.ciently 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 iu 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 scaline dodecahedrons of calcareous 
spar, and in the sujiposed altered series of rocks similar crystals are met with, 
these being generally eoroded on theu' surface, and thus affording an evidence 
of a change in the conditions existing after their formation. In connection 
with the deposits of stalactite, and in numerous small cavities in the dolomite, 
other crystals of calc s)jar are not unfrequent, but under both these circum- 
stances 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 occasionally with the plans of a 
second rhonibohedion, 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 sealeuohedral forms, and in all the cases I 
have had an opportunity of observing them, these secondary crystals invariably 
contain obtusely rhombohedral sm-faces. I may also add that there may be 
considered to be good evidence that the causes connected with the original 
formation of dolomite took place under conditions very different from those 
existin;; at the present day, for not only does the ii-on 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 phy- 
sical evidence of the presence of all the chemical compounds before described 
as occurring in them, sidjjhate of lime alone excepted ; this, it may be reujem- 
bered, I supposed to have been removed by the agency of water ; and that 
means adequate for the removal of this somewhat soluble salt existed, was 
amplv proved by the very numerous caverns produced by the decomposition of 
the dolomite to which so frequent 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 com- 
municated with a plentiful supply of water was easily proved by the splashing 
sound heard when stones were thrown into them. 
VOL. III. 2 X 
