eckel.] LIMESTONES. 29 
Silica in limestone. — Silica, when present in a limestone containing 
no alumina, may occur in one of three forms, and the one in which it 
occurs is of great importance in connection with cement manufacture. 
(1) In perhaps its commonest form silica is present in nodules, 
masses, or beds of flint or chert. Silica occurring in this form will 
not readily enter into combination with the lime of a cement mixture, 
and a cherty or flinty limestone is therefore almost useless in cement 
manufacture. 
(2) In a few cases, as in the hydraulic limestone of Teil, France, a 
large amount of silica and very little alumina are present, notwith 
standing which the silica readily combines with the lime on burning. 
It is probable that in such cases the silica is very finely divided or 
occurs as hydrated silica, which is possibly the result of chemical 
precipitation or of organic action. In the majority of cases, however, 
a highly siliceous limestone will not make a cement on burning unless 
it contains alumina in addition to the silica. 
(3) In the crystalline limestones (marbles), and less commonly in 
uncrystalline limestones, silica may occur as a complex silicate in the 
form of shreds or crystals of mica, hornblende, or other silicate mineral. 
In this form silica is somewhat intractable in the kiln, and mica and 
other silicate minerals are therefore to be regarded as inert and useless 
impurities in a cement rock. These silicates will flux at a lower tem- 
perature than pure silica, and are thus not so troublesome as flint or 
chert. They are, however, much less serviceable than if the same 
amount of silica were present in combination with alumina as a clay. 
Silica with alumina in limestone. — Silica and alumina, combined in 
the form of clay, are common impurities in limestones and are of 
special interest to the cement manufacturer. The best-known example 
of such an argillaceous limestone is the cement rock of the Lehigh 
district of Pennsylvania. Silica and alumina when present in this 
combined form unite readily with the lime under the action of heat, 
and an argillaceous limestone, therefore, forms an excellent basis for a 
Portland-cement mixture. 
Iron in limestone. — Iron when present in a limestone occurs com- 
monly as the oxide (Fe 2 3 ) or sulphide (FeS 2 ); more rarely as iron 
carbonate or in complex silicates. Iron in the oxide, carbonate, or 
silicate forms is a useful flux, aiding in the combination of the lime 
and silica in the kiln. When present as a sulphide, in the form of the 
mineral pyrite, in quantities exceeding 2 or 3 per cent, it is to be 
avoided. 
PHYSICAL CHARACTERS OF LIMESTONES. 
In texture, hardness, and compactness the limestones vary from the 
loosely consolidated marls through the chalks to the hard, compact 
limestones and marbles. They differ in absorptive properties and 
density. The chalky limestones may have a specific gravity as low as 
