644 NEW YOEK STATE MUSEUM 



dioxid. It rarely occurs perfectly pure, the impurities seldom 

 falling below 1^, while the v may increase to such an . extent as 

 to prohibit calling the rock a limestone. The impurities com- 

 monly present are silica, alumina, iron, magnesia and organic 

 matter. T-races of sulfuric acid are also met mth. 



The silica may be present either as pure quartz; combined with 

 alumina in the form of clay; or less frequently as an element 

 of silicate minerals such as mica, hornblende or pyroxene. Silica 

 may practically be looked on as an inert impurity displacing so 

 much carbonate of lime. At high temperatures, however, when 

 the carbonic acid has been driven off and oxid of lime left, the 

 silica will flux the lime with great eagerness. Alumina is usually 

 present as clay. With an increase in the percentage of the latter, 

 limestone passes into cement rock. If present to the extent of 

 only 4^ or 5^, alumina is an inert impurity like silica, but, when 

 present in larger amounts as a constituent of clay, it facilitates 

 the expulsion of the carbonic acid gas. The reason for this is 

 that clay contains chemically combined water, which passes off 

 only at a red heat or at the same time as the carbonic acid gas. 

 This pro^ddes an atmosphere of watery vapor into which the 

 carbon dioxid escapes quicker than it would if passing off into 

 gas of its own kind. 



Iron and alkalis, if present in appreciable quantity, render 

 the limestone more easily fusible, and may necessitate the hand- 

 picking of the burned rock to separate clinkers. Limestones 

 often contain appreciable amounts of magnesia. "When the 

 amount of MgO is 5fo or higher, they are called magnesian lime- 

 stones, but, when it reaches 18 fc or 20 fc, the term dolomite is more 

 frequently employed. Organic matter is rarely absent from 

 limestones, and a very small amount may impart a gray or even 

 black color to the rock. While a total of 4^ or 5^ of impurities 

 does not mean much when only a few tons of stone a day are 

 used, it becomes an appreciable item when the consumption at 

 one works amounts to 200 or 350 tons of limestone a day. 



