The ore occurs in two forms, haematites and limonites; the specular hematites often being grouped, for practical purposes, into two classes - those occurring in porphyry and those occurring in sandstone.
The haematites are found not only in the archean porphyries but in Cambrian limestone and sandstone, and in the sub-Carboniferous formations; while the limonites are confined almost exclusively to the Cambrian.
The bedded haematites and limonites have been little exploited.
The iron deposits are very extensive, and the ores consist of red haematites, magnetites, titanic, chrome and manganese irons.
These of course are the oldest of our ores, and from deposits of like age, especially those of the more readily decomposed silicates, has come the iron which now exists in the siderites and red and brown haematites of the later geological formations.