CHAPTER IV. 
ROCKS. 
WIIAT IS A ROCK ? — MATERIALS OF ROCKS — IIOW TO 
STUDY ROCKS— CLASSIFICATION OF ROCKS ROCKS OF 
SYDNEY AND THE BLUE MOUNTAINS. 
A rock is the constituent of the earth’s surface, and, 
though it may consist of a single mineral, it is 
generally a mechanical mixture of two or more 
minerals. Rocks are then, for the most part, aggre- 
gates of minerals. 
A mineral 1 is a natural inorganic substanco, which 
has a homogeneous structure, definite chemical compo- 
sition and physical properties, and usually a definite 
crystalline form. 
A mineral may consist of a single element, or 
more than one element. 
A mineral is made up of the single undecom- 
posable substances, called by chemists elements. 
These elements, in turn, form minerals, and 
minerals form rocks. 
The number of known minerals makes a long 
list, and this list is constantly increasing. The 
minerals, however, entering to any extent into the 
1 Coal is usually classed with minerals, although of organic origin, and having 
no definite crystalline form. 
