56 
Geology of Sydney. 
The cleavage of calcite is very perfect, the 
mineral breaking up into crystals always showing 
two or more perfect faces of a rhombohedron. 
Nearly all natural waters hold some carbon- 
dioxide in solution, and in that condition these 
waters can dissolve limestones. 
The vast masses of limestone that occur in the 
crust of the earth consist, for the most part, of 
Calcite or carbonate of lime. 
Iron Pyrites is a compound of Sulphur and Iron. 
Barite is a compound of the element Barium and Sulphuric 
acid. 
Gypsum is a compound of the element Calcium and Sul- 
phuric acid and some combined water. 
Limestone is a compound of Lime and a gas — Carbon dioxide. 
Lime is a compound of the element Calcium and a gas — 
Oxygen. 
8. MAGNETITE. — Magnetite is the black oxide of iron. 
It is widely diffused through rocks in grains, 
veins and beds. The mineral is strongly magnetic. 
A magnet passed through clays, arising from the 
decomposition of basalt, will usually collect some 
grains of magnetite. 
9. HAEMATITE. — Haematite is known as ferric oxide. 
Its color is black or steel grey, but the mineral 
is always red or reddish when finely powdered. 
In other words, haematite gives a red streak. The 
ironstone bands in the Hawkesbury Sandstone 
are mostly impure haematite. 
