58 
Geology of Sydney. 
Take a specimen of Sandstone, Granite, Basalt, 
and Limestone, and place these rocks side by side. 
Examine them carefully, using a pocket lens if neces- 
sary. At first sight, possibly, a beginner may think 
that there is very little to distinguish one stone from 
the other. But he soon discovers that the rocks before 
him differ in many ways in their characters, and 
that certain qualities observable in one are absent 
from others. For instance, they differ in hardness , 
color, texture, and in their composition and mode of 
origin. 
In order to classify rocks, we must decide on some 
Basis of Classification. It has been found that no 
serviceable classification can be based on the color or 
the texture of rocks. The specimen of sandstone is 
white, but sandstones are often grey and red. Basalt 
is black or blue-black, and some limestones are also 
black. Neither does a classification depending on 
texture serve any useful purpose, as sandstones, basalts, 
and granites vary much in texture. We, therefore, fall 
back on the composition and the mode of origin of 
rocks to give us a basis of classification. 
A student will, with very little trouble, note the 
following characters in the sandstone : — 
1. Sandstone is made up of numbers of small grains. 
2. The grains are held together by a cementing 
material. 
3. The grains are seen to be rounded and worn, not 
angular and sharp. 
