76 
Geology of Sydney. 
broken coral, shells, and mud washed into the inter- 
stices between the growing branches. Coral reefs are 
really great beds of limestone now in course of forma- 
tion. Vast beds of limestone are found in Australia, 
and also in the old world, consisting almost entirely 
of the calcareous joints of marine creatures known as 
crinoids or sea-lilies. See Fio-. 20. 
o 
IGNEOUS ROCKS. — If one great group of rocks 
had an aqueous origin, it is equally certain that another 
great group of rocks was formed by fire, and is 
therefore called Igneous . Some of the fire-formed 
rocks cooled and hardened deep down in the earth, 
“ 01> sometimes, perhaps, under a certain weight of 
superincumbent ocean.” 1 These are known as Plutonic 
rocks. Other fire-formed rocks were forced from the 
interior of the earth to the surface, and were poured 
out for the most part as great molten streams of lava 
from the craters of volcanoes. Basalt, usually called 
“blue metal,” is a type of volcanic rock very common 
in Australia. Great sheets of basalt are found all along 
the flanks of the dividing range, from Mount 
Kosciusko to Gape York, and every sheet is an ancient 
lava stream that poured as a river of molten rock from 
the fiery crater of some long extinct volcano. 
The Igneous rocks owe their origin to heat, they 
being at one time in a state of fusion. When this 
molten matter cooled deep down in the earth the result 
was a rock of a crystalline structure resemblinggranite, 
and rocks of this class are called Plutonic . 2 Often the 
1 Lyell’s Elements of Geology, p. 7. 
2 From Pluto, the ruler of the regions of fire. 
