On some Points in Chemical Geology

Abstract

§ I. In a paper read before the American Association at Montreal in August 1857, as also in some previous communications to the Royal Society, and in the ‘Report of the Geological Survey of Canada’ for 1856, I have endeavoured to explain the theory of the transformation of sedimentary deposits into crystalline rocks. In considering this process we must commence by distinguishing between the local metamorphism which sometimes appears in the vicinity of traps and granites and that normal metamorphism which extends over wide areas and is apparently unconnected with the presence of intrusive rocks. In the former case, however, we find thatthe metamorphic influence of intrusive rocks is by no means constant, showing that their heat is not the sole agent in alteration, while in the latter case different strata are often found affected in very different degrees; so that fossiliferous beds but little altered are sometimes found beneath crystalline schists, or even intercalated with them. We cannot admit that the alteration of the sedimentary rocks has been affected by a great elevation of temperature, approaching, as many have imagined, to that of igneous fusion; for we find unoxidized carbon in the form of graphite both in crystalline limestone and in beds of magnetic iron-core; and it is well known that these substances, and even the vapour of water, oxidize graphite at a red heat, with formation of carbonic acid or carbonic oxide. I have, however, shown that solutions of alkaline carbonates in presence of silica and earthy carbonates