70 
Geology oe Sydney. 
or calcite is a compound of lime and carbonic acid gas. 
The hydrochloric acid seizes oil the lime so eagerly 
that the other constituent is set free and escapes as 
bubbles of gas. 
Coral reefs are vast masses of limestone now in 
course of formation. A piece of compact limestone 
taken from a coral reef would answer to each of our 
tests given above. There can be no doubt, therefore, 
that limestones are formed, under water, for the most 
part by the agency of living things, and limestone is 
therefore said to be an aqueous rock of organic origin. 
We can summarize our knowledge in this way : — 
Sandstone, 
_ is made up of rounded grains, and shining spangles of 
mica. 
the grains are held together by a cement, 
the grains are quartz and do not differ from ordinary 
sand. 
the rock is arranged in layers or strata, 
the sandstone once formed a sediment at the bottom of 
the sea, and is therefore said to be a Sedimentary 
rock. 
Granite, 
is not made up of rounded grains, 
is a fire-formed or igneous rock. 
is made up of angular and partly crystalline particles 
of quartz, felspar, mica, and often hornblende, 
these minerals are interlaced and imbedded in each 
other, and hold together without a cement. 
