Dolomitic limestones

Abstract

Dolomitic limestones may have been formed in any one of many different ways. Some of these were noticed in detail. Amongst those of primary origin were (1) the various modes of precipitation connected with the deposition of strata of lacustrine origin formed in closed bodies of inland water where supply is balanced by evaporation. The materials are derived (amongst other sources) from the decomposition of the ferro-magnesian silicates. (2) Precipitation consequent upon ( a ) the dessication of lagoons, or other temporary inlets of sea water; or ( b ) upon the mingling of various solutions. (3) Deposition direct from percolating solutions derived from the decomposition of basic igneous rocks situated at higher levels. Much of this is deposited as pearl spar in veins or other fissures, but occasionally masses of more importance are formed in this way. (4) Calcareo-magnesian matters exuded in consequence of the alteration of rock-forming silicates undergoing plutonic metamorphism, are, again, another source of certain dolomites of limited horizontal extent. (5) Some dolomitic limestones may, possibly, have originated directly through the secretion of small quantities of carbonate of magnesia, along with carbonate of lime, by various organic agencies. Dolomites of secondary origin have nearly all arisen through the addition of magnesian salts—usually the carbonate—to limestones already existing as such. In some cases, as in that of The Magnesian Limestone, the rock may have been of lagoon origin, and therefore partly a magnesian precipitate, to begin with, and have subsequently received a further addition of magnesian matter through the copious downward