Limestones. 
223 
noted here that many dark -blue limestones, such as 
those that can be seen at the Jenolan Caves, polish to 
a rich velvet black. Looked at from their origin, 
these rocks are equally varied. Some limestones are 
chemically formed. Most limestones are, however, 
of organic origin — that is, the materials of which 
they are made up were separated from the sea-waters 
by the help of organised living animals. 
If we look closely at many samples of limestone, 
we cannot fail to find traces of shells, corals, crinoids, 
or other animals. Varieties of this rock are not un- 
common, consisting almost entirely of broken shells or 
the stems of crinoids. A coral reef is a mass of lime- 
stone now in course of formation. The coral polyps 
separate carbonate of lime from the sea, to form the 
stony frame-work we call coral. I he waves break 
up large quantities into masses ; these are tossed 
together, and the interstices are filled up with finer 
particles. Null i pores and other plant-like organisms 
bind the whole together. The Great Barrier Reef is 
a mass of limestone more than one thousand miles 
lone, formed in this way. The long lines of limestone 
we find interbedded in Silurian rocks are often 
nothing more than old coral reefs. The corals are 
certainly not the species that form reefs to-day , but, in 
origin there is no essential difference. Our Austra- 
lian coral limestones give evidence of a shallow water 
origin. We have, however, a group of limestones that 
were undoubtedly formed under conditions that may 
be termed oceanic. The chalk of Europe is usually 
