dation, so that the plants are able to use over again the carbon dioxide; but this is a subordinate action in plants, in animals it is the condition of their existence, and all animals are breathing out carbon dioxide continuously. The amount of carbon dioxide given off by animals in the process of respiration, added to the artificial production of carbon dioxide in the burning of fuel, is, however, insignificant as compared with the amount poured out by volcanoes.
Another means for abstracting carbon dioxide from the atmosphere is that of limestone formation. Vast numbers of animals, such as shell fish, corals, and so forth, and a few plants, have the power of secreting their carbon dioxide combined with lime, so that the carbon dioxide is not returned to the atmosphere but is laid by indefinitely. It follows, therefore, that a volcanic epoch is one in which the atmosphere is being supplied with a superabundant amount of carbon dioxide, whereas a period of intense limestone formation, as happened when the chalk of England was formed, or a period of coal formation, means an excessive abstraction of carbon dioxide from the air. The one follows the other, and in past geological ages the volcanic epoch has been followed by tropical conditions over the whole globe, so that trees such as the cinnamon and magnolia have been enabled to grow within the Arctic circle, and forests have flourished in the Antarctic; whereas the limestone periods and the coal periods have been followed by glacial conditions. The Carboniferous Period, when most of the great coal seams of the world, in America, Europe, and China, were formed, was followed in the Southern Hemisphere at least, certainly in South Africa, by an Ice Age, when the streams became frozen and glaciers filled the valleys.
