eckel.] LIMESTONES. 27 
VARIETIES OF LIMESTONE. 
A number of terms are in general use for the different varieties of 
limestone, based upon differences of origin, texture, composition, etc. 
The more important of these terms will be briefly defined. 
The marbles are limestones which, through the action of heat and 
pressure, have become more or less distinctly crystalline, though the 
term marble is often extended to cover any limestone which will take 
a good polish. The term marl, as at present used in cement manufac- 
ture, is applied to a loosely cemented mass of lime carbonate formed 
in lake basins, as described on page 34. Calcareous tufa and traver- 
tine are more or less compact limestones, deposited by spring or 
stream waters along their courses. Oolitic limestones, so called 
because of their resemblance to a mass of fish roe, are made up of 
small rounded grains of lime carbonate having a concentrically lami- 
nated structure. Chalk is a tine-grained limestone composed of finely 
comminuted shells, particularly those of the foraminifera. The pres- 
ence of much silica gives rise to a siliceous or cherty limestone. If 
the silica present is in combination with alumina the resulting lime- 
stone will be clayey or argillaceous. 
CHEMICAL COMPOSITION OF LIMESTONE. 
A theoretically pure limestone is merely a massive form of the min- 
eral calcite. Such an ideal limestone would therefore consist entirely 
of calcium carbonate or carbonate of lime (CaC0 3 ) or 50 per cent cal- 
cium oxide (CaO) plus 44 per cent carbon dioxide or carbonic acid 
(C0 2 ). As might be expected, limestones as quarried differ more or 
less widely from this theoretical composition. These departures from 
ideal purity may take place along either of two lines: (1) The presence 
of magnesia in place of part of the lime; (2) the presence of silica, 
iron, alumina, alkalies, or other impurities. 
It seems advisable to discriminate between these two cases, even 
though a given sample of limestone may fall under both heads. 
MAGNESIA IN LIMESTONE. 
The theoretically pure limestones are, as above noted, composed 
entirely of calcium carbonate and correspond to the chemical formula 
CaC0 3 . Setting aside for the moment the question of the presence or 
absence of such impurities as iron, alumina, silica, etc., it may be said 
that lime is rarely the only base in a limestone. During or after the 
formation of the limestone a certain percentage of magnesia is usually 
introduced in place of part of the lime, thus giving a more or less 
magnesian limestone. In such magnesian limestones part of the cal- 
