220 
Geology of Sydney. 
Calcium carbonate, a compound forming the greater 
bulk of all limestones. White crys- 
talline limestone is almost pure 
calcium carbonate, more com- 
monly called carbonate of lime. 
Calcium oxide is ... “ quick-lime, ; ” and is made by 
burning limestone. 
Calcium hydrate is “ slaked lime,” and is made by 
pouring water on “ quick-lime.” 
Shells and Coral ... consist almost entirely of car- 
bonate of lime. 
When “ lime” is spoken of in every-day language. 
Calcium oxide of the chemist is referred to. 
Limestone is one of the few soluble rocks forming 
considerable rock masses. The simplest test for lime- 
stone is treatment with a few drops of hydrochloric 
acid, a brisk effervescence telling at once if a rock is 
limestone or markedly calcareous. Strictly speaking, 
the test only shows that a soluble carbonate is present. 
But as limestone is the only commonly occurring rock 
that is a carbonate, the test is satisfactory. This 
simple test is made use of even by skilled geologists, 
as limestone varies much in appearance — pure snow- 
white and every shade through grey and brown to 
dead black being known. Some limestones are also 
brightly coloured. In texture, also, there is a great 
diversity, many limestones being fine-grained and dull, 
others being coarsely crystalline. Marble is only a 
variety of limestone, and usually any limestone hard 
enough to take a polish is called marble. It may be 
