Geology oe Sydney. 
burning the coal it comes in contact with in its 
upward course. There can be no doubt basalt was 
once molten, and that it flowed from some old and 
perhaps long extinct volcano. We are now able to 
say that Basalt is a fire-formed or igneous rock, 
which welled up through some crack in the earth s 
crust or flowed from a volcano, and is an intimate 
aggregate of several minerals. 
We take the limestone next, and may bear in 
mind that we are dealing with one of the most in- 
teresting and important of all rocks. Limestone is a 
greyish or bluish rock, sometimes white, sometimes 
black or brown. The texture is fine and close grained, 
but may show a crystalline structure not unlike loaf- 
sugar. On examining a number of specimens you are 
sure to find embedded in the stone some remains or 
traces of sea-living animals. As a rule, a general 
examination will tell us nothing more. The chemist 
and the geologist come to our assistance here, and we 
are told that chalk, marble, and common limestone are 
mere varieties of the same stone, and that they are of 
the same chemical composition as sea shells and coral. 
Let the student now take a piece of coral or a sea 
shell, and note that both can be scratched with a knife, 
giving a white powder. Lift a drop of strong 
hydrochloric acid 1 on a glass rod and carefully transfer 
the acid to the shell. Observe that the acid effervesces 
1 Hydrochloric acid must he handled with caution and kept in a ghss-stoppered 
bottle, labelled poison. It is very destructive to clothes, and a valuable lesson will 
be learned by trying the effect of a few drops of the acid on a worn out coat. 
