PIODGE — OSSIFEROUS CAVERNS AT ORESTON. 



345 



quent occurrence in tliera, the beds containing occasionally iron pyrites, which, 

 by the action of the weather, tinge the surfaces of the argillaceous and cal- 

 careous 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 com- 

 pounds, 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 our 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 analysis 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 scaline dodecahedrons of calcareous 

 spar, and in the supposed altered series of rocks similar crystals are met with, 

 these being generally coroded on their 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 spar are not unfrequent, but under both these circum- 

 stances they exhibit different forms, those of the stalactite being generally acute 

 rliomboliedrons, whilst the dolomitic cavities are lined with crystals having the 

 figure of obtuse rhombohedrons, combined occasionally with the plans 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 

 liave had an opportunity of observing them, these secondary crystals invariably 

 contain obtusely rhombohedral surfaces. 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 

 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 phy- 

 sical evidence of the presence of all the chemical compounds before described 

 as occurring in them, sulphate of lime alone excepted ; this, it may be remem- 

 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 

 amply 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 tin-own into them. 



VOL. III. 2 X 



