Although the calcium oxide and magnesia of sedimentary limestone and dolomite must all be derived from pre-existing igneous rocks, it is not so easy to trace the origin of the necessary carbon dioxide. The solidified igneous rocks contain mere traces of free carbon dioxide and of calcite. During their weathering carbon dioxide is taken up from the atmosphere, replacing the silica of calcium silicate and magnesium silicate minerals, and forming the carbonates. Now, however, the quantity of carbon dioxide retained in the atmosphere is exceedingly small. It is difficult to credit, for instance that a layer of coal, 1½ millimetres thick, covering the surface of the globe, on being burnt would yield as much carbon dioxide as the entire atmosphere now contains. A similar layer of limestone containing an equivalent amount of carbon dioxide would have a thickness of 5 millimetres. This is out of all proportion compared with the mighty beds of limestone and dolomite now forming part of the earth's crust. Nor would the carbon dioxide of the sea avail anything. It exceeds in quantity that of the air, but is still at the most equivalent to 7 centimetres of limestone covering the earth's surface. Now the existence of animal life throughout the sedimentary era implies that from the earliest times, the composition of the earth's atmosphere cannot have differed very materially from the present one.