It was held not long ago that the igneous metamorphism of a limestone involved no notable additions of material. This view is now generally abandoned, and it is recognized that the most far-reaching changes may take place involving very large additions of silica, as well as iron and other metals. It is also recognized that the limestone usually suffers a loss not only of carbon dioxide, but also of calcium oxide and sometimes other constituents. This has been abundantly proved by changes which alter a pure limestone into a heavy iron-lime silicate, such as andradite, and it has been shown that the volume, aside from pore space, remains substantially the same.
In case of an impure limestone the difficulty is greater because the average composition of the bed is rarely known and the exact additions and subtractions that have taken place remain more or less obscure.
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