HODGE— OSSIFEROUS CAVERNS AT ORESTON. 



377 



ON THE OSSIFEROUS CAYERNS AT ORESTOK 



By Henry C. Hodge. 



(Continued from page 344, vol. iii.) 



In concluding this paper, allow me to remark that if the deductions to which 

 I have arrived be correct, some data will have been afforded for explaining, 

 through the agency of analogous chemical changes and their resulting products, 

 the cause of some, at least, of the various distinctive characters presented by 

 those 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 manner, 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 

 important bearing the chemical changes described may be presumed to have on 

 the solution of the 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 frec^uent result of 

 the decomposition of its dolomite, and that sandy beds of a similar kind are 

 also not unfrequent in the limestone itself. If we call to mind the fragmentary- 

 condition of the fossils of the Old Red Sandstone strata, it may not be 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 

 have 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 

 the luxuriant 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 Hme, under the reacting influence of the 

 organic matter, (assisted by the high 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 still 

 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 

 wdtli other salts and carbonates (together with alkaline chlorides, from their 

 solvent action on the substance of decaying plants), viz., bicarbonates of iron 

 and magnesia with those of lime, such depositing with admixed clayey mud : — 

 rirstly, those valuable argillaceous 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 readily to 

 be obtained from the decomposition of pyrites, now form beds of gypsum,' and 

 with sand the New Red Sandstone, and the remaining waters, now almost 

 divested of their lime, proceed to dolomize the submarine calcareous rocks, 

 efi^acing thus in part the record of their first existing organisms. 



VOL. III. 3 B 



