ROCKS | 69 
largely concerned in the formation of calcareous tufa, which 
is not always due entirely to the mere evaporation of aqueous 
solutions. The carbonic acid which enables the water to 
hold the calcium-carbonate in solution is decomposed by bog- 
mosses and their allies, and a calcareous crust is thus 
gradually deposited upon the plants. Many thick masses of 
tufa have in this way resulted partly from chemical, and 
partly from organic action. Considerable accumulations of 
siliceous sinter are likewise due in large measure, as already 
indicated, to the vital action of minute alge. But of still 
greater importance are the rocks which owe their formation 
to the action of humus acids derived from the decomposition 
of organic matter. These acids attack the iron-bearing 
mineral constituents of rocks, and form ferruginous solutions ; 
and when such solutions are exposed to the air, they are 
oxidised, and hydrate of iron (bog iron-ore) is precipitated. 
There can also be little doubt that organic acids, derived from 
the decomposition of sponges and other forms of life, have 
had much to do with the formation of flint and chert, which 
might, therefore, be included among organically derived rocks 
as fitly as under chemically formed rocks. The most char- 
acteristic representatives of the former class, however, are 
the calcareous and carbonaceous rocks, of which there are 
many varieties. 
Limestone.—Of this rock there are innumerable kinds. 
All are composed essentially of carbonate of lime, but few do 
not also contain carbonate of magnesia. While some are 
very pure, others are crowded with impurities. They vary 
greatly also as regards texture—ranging from extremely 
fine-grained and compact rocks to coarse aggregates of shells 
and corals. Grey and greyish-blue are the commonest 
colours, but there are many others, such as green, purple, 
yellow, red, grey, black, and pure white. While limestones 
differ as regards their hardness and specific gravity, the 
common and most characteristic types have usually a 
hardness of 3. or thereabout, and a specific gravity of 2-6 to 
2-8. One of the best known limestones is common C/alk—a 
white fine-grained earthy rock, generally soft and meagre to 
the touch, soiling the fingers. It is largely composed of the 
shells of foraminifera, together with the débris of various 
