Rocks. 
75 
springs, rising from great depths, contain a large 
quantity of mineral matter in solution. A considerable 
part of this material (carbonate of lime chiefly) is 
deposited when the water reaches the surface. If by 
any means the carbonic acid gas is removed or broken 
up, a further quantity of the dissolved carbonate of 
lime is deposited, and, if the water should evaporate, 
the whole of the dissolved materials will remain and 
settle down on the bottom, layer upon layer. Fresh- 
water limestones are formed in this way near Bun- 
goriia, and near Cooma, New South Wales. Stalactites, 
that form so marked a feature in limestone caverns, 
are familiar examples of Aqueous rocks formed by 
chemical agency. Water with carbonate of lime in 
solution drops from the roof of a limestone cave. 
Each drop, as it falls to the floor, partially evaporates, 
and leaves a thin crust of limestone, which grows a 
little with every drop, so that in time a column of 
this carbonate of lime or limestone rises from the 
floor to meet a pendant of the same material, formed 
in a similar way on the roof of the cavern. 
A third class of Aqueous rocks is formed from 
the fragments of plants and animals, and is therefore 
said to be organic. Coal is a rock composed for 
the most part of vegetable remains. Chalk, which 
occupies great areas in Europe, is made up almost 
entirely of foraminiferal shells, so minute as to be 
visible only under the microscope. A common variety 
of limestone has been formed by the continuous growth 
of corals, cemented into a solid rock by fragments of 
