METAMORPHISM. 769 



the concentration it would not take place; and hence variations in such concentration 

 may account for the differences in composition that occur between the successive layers 

 of some magnesian limestones, or, the alternation of magnesian and non-magnesian 

 layers, or the occurrence of dolomitic fossils when the rock is purely calcareous ; fossils, 

 as numerous facts show, being the h'rst part to become changed. 



According to one of the methods of forming dolomite, suggested by T. S. Hunt, cal- 

 cium bicarbonate and magnesium sulphate in solution, react on one another, and in the 

 double decomposition produce magnesium bicarbonate and calcium sulphate (gypsum), 

 and then, on evaporation, a deposit of the gypsum takes place, and if the supply of the 

 calcium bicarbonate continues, " the two carbonates fall in a state of intermixture." 

 But it is not yet shown that dolomite thus results; and the theory does not meet the 

 case of the Metia coral-reef rock, since this was not a chemical precipitate, but, like 

 other great limestones, it was of organic origin. 



Besides dolomite, originating in the manner above mentioned, there h other dolomite 

 which is a result of the chemical alteration of beds of true limestone. Adolf Schmidt 

 has pointed out (Trans. St. Louis Acad., 1875) that, at the large lead aud zinc deposits 

 in the Subcarboniferous limestone of Missouri, the limestone has been rendered dolo- 

 mitic along fissures and about the ores; and he refers this dolomizahon — following 

 Bischof — to the action of "solutions of magnesium bicarbonate." Limestones have 

 been found to have in many regions transverse bands of dolomite crossing the bedding 

 along the courses of joints or fractures; and the formation of the dolomite has been 

 attributed to the action of solutions of magnesium carbonate by some, but to the mag- 

 nesium salts of sea-water by Prof. Harkness, who describes some examples in the 

 Carboniferous limestones of Ireland. Crystals of calcite altered to dolomite, have been 

 described by various authors. 



In a memoir on the famous dolomite region of the Tyrol, Dolter and Homes, geol- 

 ogists of Vienna, discuss this subject at length, and reach the following conclusions: 

 (1) Some large limestones, weakly dolomitic, may have been made out of those organic 

 secretions which contain a little magnesia ; (2) minor cases of the production of dolo- 

 mite are due to the alteration of limestone through the introduction of magnesium car- 

 bonate; but (3) the larger part of dolomite formations, whether more or less rich in 

 magnesia, have been formed from organic calcareous secretions through the action of 

 the magnesium salts of sea-water, especially the chloride. 



The following are titles of some works and memoirs bearing on the subject of meta- 

 morphism : — 



G. Bischof: Lehrbnch der Chemischen nnd Physikalischen Geologie; 3 vols. 8vo, 

 2d ed., Bonn, 1863-1866. 



J. Roth : Allgemeine und Chemische Geologie: 1st vol. on the Formation and Altera- 

 tion of Minerals and on mineral and marine waters, 8vo, Berlin, 1879 ; a full and sys- 

 tematic exposition of the known facts. 



C Lyell: Principles of Geology, 1st ed., 1833, London. First use of the term meta- 

 morphic rocks ; also first application of the term hypogene rocks to granite, gneiss, and 

 other crystalline kinds, supposed to be nether-formed rocks. 



S. P. Sckope : See page 748. 



C. Barrage : See page 722. 



Scheehek: Aqueo-igneous Origin of Granite, etc. Pogg. Ann., lxviii., 319,1846. 

 Bull. Soc Geol., France, II., iv., 408, 1847. Ninth Bridgewater Treatise (1851), 225* 



H. C. Sokby: Microscopical Structure of Crystals, indicating the origin of minerals 

 and rocks, Quart. J. Geol. Soc, xiv., 453, 18?8, and Reports Brit. Assoc, 1856, p. 78; 

 1857, p. 92; shows, by careful experiments, that microscopic cavities in crystals depos- 

 ited from solutions in water, contain water; that the size of the vacuities in cavities 

 depends on the temperature and pressure; that those in crystals formed from igneous 

 fusion contain glass or stone; that those in crystals made under great pressure, by the 

 combined action of igneous fusion and water, or by igneo-aqueous fusion, may contain 

 either water alone, or glass or stone; and thence deduces, among other results, that the 



