IRON MANUFACTURE. : 487 
frequently the mass has hard curling bands running through it which 
give rise to the name of “curly ore.” The red limestone ore is quite 
pure, containing on an average about 10 per cent. of silica, and tne 
amount of lime and alumina is never very large in the clean ore. 
Manganese is present, on an average, something more than 14 per cent., 
which, however, is scarcely a proportion large enough to affect the 
quality of the iron directly. Sulphur is present in very small quantities, 
and in the mean of many analyses rarely exceeds ;35 of one per cent. 
Phosphorus, however, is most always present, and is the most consider- 
able of the damaging impurities, there being on an average about 75 of 
one per cent. 
The “red limestone” ore extends into the hills to a depth depend- 
ing upon its position and the character of the overlying rocks as regards 
their imperviousness to the action of water, and passes into the un- 
changed carbonate ores, which was*probably%,the original condition of 
the entire stratum. The extent of fthe change varies considerably, as 
sometimes the limonitic or red limestone variety prevails, while at other 
times the change has not been so extended and the carbonates are more 
abundant. The mining operations, however, having been mostly con- 
fined to the outcrops of the seam, the carbonate has always been obtained 
in smaller proportions than the oxidized or “red limestone” part. This 
ore is a carbonate more or less calcareous, and the analyses made con- 
tain rarely more than 8 or 10 per cent. of the carbonates of lime and 
magnesia, and in the clean ore but very small proportion of alumina. 
Very frequently it passes almost imperceptibly into quite a pure lime- 
stone containing scarcely any iron at all. It is found in two varieties 
—the oray and blue limestone ores. The gray limestone cre is some- 
what odlitie in character, being formed of a mass of small granules of 
carbonate of iron cemented together by a calcareous and silicious 
cement, which, on exposure to the weather, softens and crumbles. 
When burnt, however, the carbonate is oxidized, the mass becoming of 
a brick-red color, and though being more difficult to caleine than the 
other ores, is highly valued for the ease with which it works in the 
furnace and the character of the iron produced. The blue limestone 
ore is a more uniform calcareous ore of the limestone seam, quite hard 
and compact, and not unfrequently passing into a limestone containing 
little or no iron. These unchanged “limestone” ores vary somewhat 
in the iron contained, which is by analysis from 28 to 30 per cent., the 
