On some abnormal deposits of limestone

Abstract

The author reviewed several of the different modes whereby limestones may have been formed, his chief conclusion being that a variable, but often large, percentage of the constituent matter of nearly all limestones is of the nature of a chemical precipitate, due to the reactions of matters of organic origin upon the sulphate of lime in sea-water. [When the paper was read the author advocated the view that the carbonate of lime secreted by aquatic animals—and therefore the chief organic constituents of limestones—was obtained by the organisms from the sulphate of lime existing in solution in sea-water, and he gave several reasons for doubting whether any of the carbonate of lime of organic origin could have been, in any case, assimilated direct. The author was not aware until after the paper was read, that these points had been already demonstrated by Messrs Irvine and Woodhead. (Proc. Roy. Soc. Edin., vol. xvi., p. 324.) The author had independently arrived at this conclusion by an entirely different line of reasoning.] Some reference was made to limestones that are entirely of inorganic origin. It was pointed out that where basic igneous rocks are undergoing decomposition, or other changes in composition, carbonate of lime is one of the substances liberated; and this is often redeposited at lower levels. As a rule such deposits consist of calcspar and are more or less crystalline; but in some few instances the mineral is left nearly in an amorphous form, hardly, if at all, distinguishable from sedimentary limestone. Some well-banded and fine textured limestones traversing the bedding planes of the volcanic rocks on the shore east of North Berwick appear to be of this nature. Other illustrations were referred to. More commonly the decomposition of the ferro-magnesian silicates in rocks of basic composition gave rise to beds of native dolomite. Some examples were adduced.