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 frequent 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 considered unreasonable to suppose that they have been formed from the decomposition of rocks similar to those of our Devonian limestones, in which iron pyrites was much more abundantly distributed.