52 
Geology of Sydney. 
composition of the rocks we have to deal with, are not 
many. 
1. QUARTZ. — Quartz is an oxide of silicon ; in other 
words, it is a substance made up of the two 
elements Oxygen and Silicon. 
The milk-white stone of a quartz reef, the clear 
glass in pebble spectacles, and the grains of sand 
on the sea-shore, or the grains that go to make 
up the Hawkesbury Sandstone, are chemically 
one and the same substance. 
Quartz is very hard ; cannot be scratched with 
a knife, but quartz scratches glass easily. It is a 
most insoluble substance, but is attacked slowly 
by heated caustic alkalies, and by hydrofluoric 
acid. The reddish-looking quartz in some quartz 
reefs is due to staining by oxide of iron 
Amethyst, a gem stone we prize considerably, is 
quartz colored by oxide of manganese. Cairn- 
gorm is also a colored quartz. Chalcedony, Opal, 
Agate, Flint, Jasper, are all varieties of quartz. 
2. FELSPAR. — The varieties of felspar are many, but 
they are all slicates of alumina with potash, 
soda, or lime added. The felspars decompose 
rather easily, giving rise to Kaolin. The finest 
pottery is made from Kaolin, Kaolin itself being 
merely a decomposed felspar. 
The lath-shaped crystals seen in thin slices of 
Basalt from Bathurst are felspars. The glisten- 
ing rectangular crystals in the Bowral Syenite 
(Trachyte of the builders) are also felspars. 
