PRELIMINARY REMARKS. 
41 
silicates and of fine and impalpable sands. Their purity or 
approach to alnmine is indicated by their whiteness. 
Limestone is composed of carbonic acid and lime. As mar¬ 
ble, it is nearly pure. When acid is poured on limestone it 
effervesces or boils by the escape of carbonic acid. Lime is 
also found combined with sulphuric acid, when it forms gyp¬ 
sum. It may be distinguished from the carbonate by its soft¬ 
ness, its fusibility, and the absence of effervescence in the pre¬ 
sence of acids. 
The sediments are mixtures of the silicates, sand or quartz, 
in fine or coarse grains, pebbles, clays, sandy clays, limestones, 
or sandy and aluminous limestones. These mixtures, however, 
never form chronological successions; neither do they occur in 
modes or ways by which their lithological properties may be 
used as characteristics of age or place. To say that a rock is 
limestone, sandstone, or slate, conveys no idea of its place. It 
is a mineralogical fact which has some importance. 
Among the chemical and mechanical mixtures iron is rarely 
absent. Its presence is usually indicated by red and brown 
colors, which it imparts to the mixtures containing it. The 
red and brown sandstones are stained with it. Of the simple 
bodies, however, oxygen must be regarded as the most general, 
and most widely diffused in the mineral kingdom. In a state 
of purity it is aeriform. Its properties are better known to us 
in its mixture with nitrogen, forming the atmosphere. Very 
few substances are known which do not contain it. The iron 
which has just been referred to, is a compound of iron and oxy¬ 
gen, or it is an oxide of iron. Sulphuric acid, which forms a 
part of gypsum, is a combination of sulphur and oxygen; and 
carbonic acid a combination of pure carbon and oxygen; quartz 
or flint is silicon and oxygen; and quick lime is calcium and 
oxygen. 
§ 36. It is a point upon which all geologists agree, that the 
earth’s crust is composed of rocks which have been formed at 
different periods. Both the rocks and periods being numerous, 
it is important they should be arranged into groups or classes, 
