Rocks. 
69 
or “ boils up ” freely. Place a drop of the acid on 
granite ; there is no effervescence. Place a drop of the 
acid on your specimen of limestone, and see that it 
effervesces just as in the case of the shell. 
There must be something in common then between 
shells, coral, and limestone. The fact is, that all three 
are composed of calcite. 
Fio. 20.— A piece of limestone made up almost entirely of the 
remains of marine animals. Rocks of this kind are com- 
mon in Australian Silurian Limestones, as at Cave Creek, 
Orange, New South Wales. 
From a geological standpoint calcite is a mineral 
of the first importance, being tbe sole essential con- 
stituent of all limestones. Calcite is the only common 
mineral effervescing freely with cold hydrochloric acid, 
and limestone is the only rock that will effervesce freely 
with cold hydrochloric acid. This behaviour with 
hydrochloric acid is a geologist’s rough test for lime- 
stone, marble, and calcite. Why these rocks effervesce 
with the acid is simple enough. Chemically, limestone 
