line in structure, are not really marbles in the technical sense of the term, but are limestones that are sufficiently dense and hard to take a polish, due to the percentage of their basic matter which is present as Calcite rather than as primary calcium carbonate.
The line between marble and limestone is therefore a difficult one to draw, as the two merge into one another, and it is more nearly correct when designating monotone or neutral-toned, semi-crystalline calcareous building stones, to base the application of these terms on the trade usage of the
material. The term “marble” being reserved for decorative stones, used principally for interior work, and the term “limestone” for that material used for general building purposes, principally exteriors, but including interior work in which the texture, natural color-tone and carved or sculpture detail is the principal feature or a factor governing its usage, rather than the colors and veining or other markings developed by polishing or honing of plain or moulded surfaces.
Having thus explained the close relation of certain limestones with certain classes of marble, we will now proceed further to consider limestone and the several general classes into which it is divided.
In a previous paragraph, it was stated that limestone was composed principally of the mineral “calcite” or “calcium carbonate.” There is a varying exception to this, in that stones composed principally both of calcium carbonate and magnesium carbonate are also classed as limestone, when the proportion of the magnesium carbonate (Mg C03) replacing the calcium carbonate (Ca C03), does not exceed a given proportion, since practically all limestone will have some small percentage of magnesium in its composition. When the magnesium carbonate does not exceed 10 per cent the stone is classed as limestone; when it exceeds that percentage, as Magnesium Limestone; and when it equals 30 per cent or more, as Dolomite.
Dolomite and Magnesium Limestones are not very generally used in the production of cut stone for building purposes, hut this class of limestone rock is extensively used for burning into quick lime (lump lime) and for the production of hydrated lime for building and other purposes. Dolomitic marbles on the other hand are quite frequently used for ornamental work.
Composition of Limestone
Carbonate of lime is the principal constituent of shells and of corals, and it is largely to the accumulation of the shell remains of small to minute shell-bearing marine organisms, including corals, more or less broken and ground up into fragments of fairly uniform size by the action of the waves or currents, that limestones owe their origin.
Thus a limestone rock is formed of calcium carbonate generally in the form of calcareous sand grains cemented together with the same material, rubbed off of other shells or shell grains and dissolved by the water action, and then again redeposited from solution. Whereas sandstone is composed of quartz or silica sand grains, cemented or consolidated in various ways.
Many limestones and especially those most useful for building purposes may therefore be referred to as calcareous-sand rock. The Indiana Limestone or so-called “Bedford stone” is the outstanding domestic example of this formation, and will be described later.
The shells, when incoiporated in the stone in their original form rather than as fragments, are known as fossils. Apart from the differences in composition already mentioned, the different varieties of limestone owe their difference principally to: first, their purity and freedom from an appreciable percentage of other mineral elements ; second, to the form of aggregate or degree of fineness to which the shell fragments are reduced; third, the thoroughness of consolidation and cementing together of the grains; fourth, the freedom from interbedding of foreign matter in seams and consequent layering or stratification of the formation; and fifth, to the degree that crystalline
Fig. 7. Baptismal Font in St. Paul’s Cathedral, Toronto,
carved of limestone
The line between marble and limestone is therefore a difficult one to draw, as the two merge into one another, and it is more nearly correct when designating monotone or neutral-toned, semi-crystalline calcareous building stones, to base the application of these terms on the trade usage of the
material. The term “marble” being reserved for decorative stones, used principally for interior work, and the term “limestone” for that material used for general building purposes, principally exteriors, but including interior work in which the texture, natural color-tone and carved or sculpture detail is the principal feature or a factor governing its usage, rather than the colors and veining or other markings developed by polishing or honing of plain or moulded surfaces.
Having thus explained the close relation of certain limestones with certain classes of marble, we will now proceed further to consider limestone and the several general classes into which it is divided.
In a previous paragraph, it was stated that limestone was composed principally of the mineral “calcite” or “calcium carbonate.” There is a varying exception to this, in that stones composed principally both of calcium carbonate and magnesium carbonate are also classed as limestone, when the proportion of the magnesium carbonate (Mg C03) replacing the calcium carbonate (Ca C03), does not exceed a given proportion, since practically all limestone will have some small percentage of magnesium in its composition. When the magnesium carbonate does not exceed 10 per cent the stone is classed as limestone; when it exceeds that percentage, as Magnesium Limestone; and when it equals 30 per cent or more, as Dolomite.
Dolomite and Magnesium Limestones are not very generally used in the production of cut stone for building purposes, hut this class of limestone rock is extensively used for burning into quick lime (lump lime) and for the production of hydrated lime for building and other purposes. Dolomitic marbles on the other hand are quite frequently used for ornamental work.
Composition of Limestone
Carbonate of lime is the principal constituent of shells and of corals, and it is largely to the accumulation of the shell remains of small to minute shell-bearing marine organisms, including corals, more or less broken and ground up into fragments of fairly uniform size by the action of the waves or currents, that limestones owe their origin.
Thus a limestone rock is formed of calcium carbonate generally in the form of calcareous sand grains cemented together with the same material, rubbed off of other shells or shell grains and dissolved by the water action, and then again redeposited from solution. Whereas sandstone is composed of quartz or silica sand grains, cemented or consolidated in various ways.
Many limestones and especially those most useful for building purposes may therefore be referred to as calcareous-sand rock. The Indiana Limestone or so-called “Bedford stone” is the outstanding domestic example of this formation, and will be described later.
The shells, when incoiporated in the stone in their original form rather than as fragments, are known as fossils. Apart from the differences in composition already mentioned, the different varieties of limestone owe their difference principally to: first, their purity and freedom from an appreciable percentage of other mineral elements ; second, to the form of aggregate or degree of fineness to which the shell fragments are reduced; third, the thoroughness of consolidation and cementing together of the grains; fourth, the freedom from interbedding of foreign matter in seams and consequent layering or stratification of the formation; and fifth, to the degree that crystalline
Fig. 7. Baptismal Font in St. Paul’s Cathedral, Toronto,
carved of limestone