llODGIi — OaSlFEKOUS CAVEKNri AT OKK.STON. 
377 
ON THE OSSIFEROUS CAVERNS AT ORESTON. 
By Henry C. Hodge. 
(Contmucil fro7ii ])ar/c 344, vol. iii.) 
In concludiii!? iliis paper, allow me to remark tliat if the deductions to wliicli 
I liave arrivecl l)e correct, some data will have bceu afforded for expliiiiiing, 
tlirougk the agency of analogous chemical changes and their resulting products, 
tlic cause of some, at least, of the various distinctive characters presented by 
tliosc rocks which constitute that portion of the earth's surface formed from 
the decomposition of previously existing rocky masses. I will not, however, 
take up your time with any lengthy arguments to strengthen my position in 
such a mamier, but will merely attempt to give a very brief description of what 
I conceive we may not unreasonably infer has taken place during past ages of 
the world's history, remarking on the various geological formations in the 
general order of their occurrence ; and, firstly, I would direct attention to the 
nnportant bearing the chemical changes described may be presumed to have on 
the solution of tl:e question as to the geological equivalence of the Old Red 
Sandstone rocks of Scotland to those of our Devonian era. I have before 
adverted to the occurrence of red sand in a decomposed slaty seam of the 
Plymouth limestone, and would add that such red sand is a frequent result of 
tlie decomposition of its dolomite, and that sandy beds of a similar kind are 
also not unfrequent in tlie limestone itseK. If we call to mind the fragmentary 
condition of the fossils of the Old Red Sandstone strata, it may not oe consi- 
dered unreasonable to suppose that they have been formed from the decom- 
position of rocks similar to those of our Devonian limestones, in which iron 
pyrites was much more abundantly distributed. If this be true, there must 
nave been generated at this period an enormous quantity both of carbonic acid 
and of sulphate of lime — the former no doubt required for the sustenance of 
tlie luxui'iaut vegetation of the succeeding coal period, and a most active agent 
in producing similar chemical changes to those I have just now endeavoured 
to explain — and the sulphate of Ume, under the reacting iutluence of the 
organic matter (assisted by the hie;h temperature of that period), being changed 
to sulphide of calcium, and simultaneously, through the influence of the car- 
bonic acid, again becoming resolved into carbonate of lime — changes stiU 
traceable in the waters of the present day. At the same time, too, with the 
formation of this carbonate of lime, and the presence of such large amounts of 
carbonic acid, it is reasonable to conclude that the waters might be charged 
with other salts and carbonates (together with alkaline chlorides, from their 
solvent action on the substance»of decaying plants), viz., biearbonates of iron 
and magnesia with those of lime, such depositing with admixed clayey mud : — 
Firstly, those valuable argUlaceous carbonates of iron of the coal measures, 
and the immense excess of carbonic acid contributing to undermine the founda- 
tions of the rocks, and interstratify them with the coal ; after this depositing 
the mountain limestone, and the resulting fluid from these products readUy to 
be olDtained from the decomposition of pyrites, now form beds of gypsum, and 
with sand the Xew Red Sandstone, and the rernainuig waters, now almost 
di\ csted of their lime, proceed to dolomize the subraarme calcareous rocks, 
clfacing thujs in part the record of theii' first existing organisms. 
VOL. III. 3 B 
