Granite and all similar crystalline aggregates of silicates disintegrate, and the separate minerals are decomposed chemically by the action of carbon dioxide and moisture. Of the various compounds which result, those of carbon dioxide with lime and magnesia are of most direct interest in this connection, and those with lime may be discussed as representative.
The common combinations of lime and carbon dioxide are two: the carbonate of lime, more specifically called the normal or monocarbonate, and the bicarbonate of lime. The carbonate consists of one ion, or chemical unit, of lime, CaO, combined with one ion of carbon dioxide, CO2. The bicarbonate consists of one ion of lime combined with two ions of carbon dioxide. The carbonate is but slightly soluble in water, the bicarbonate is easily dissolved. The carbonate is produced in the decomposition of silicates, and great amounts of it which have been derived from this source in past ages are now contained in limestones and other calcareous sedimentary rocks. Whether it exists for a brief time in the weathering of silicates or is, as limestone, exposed to atmospheric waters, the carbonate very readily combines with carbon dioxide, and the bicarbonate is formed in solution. All surface and underground waters contain bicarbonate of lime in greater or less quantity, and enormous volumes are annually conveyed to the sea. It is estimated roughly that the weight of the carbonate of lime thus dissolved and contributed to the sea annually is 2,700,000,000 tons. This is about one-half of the total saline matter dissolved in surface waters annually, and a portion of the remainder consists of carbonates of magnesia, potash and soda.
It has been computed by Professor Chamberlin and his associates that the present supply of carbon dioxide in the atmosphere would be exhausted by the decomposition of silicates in 5,000 to 18,000 years at the present rate of consumption if there were no source of replenishment. It is evident that the amount of carbon dioxide in the atmosphere at any time is the balance between supply and draft, and that it may be more or less as one or the other preponderates. The next step in forming the hypothesis, therefore, is to consider conditions which may produce fluctuations of consumption and contribution.
The consumption of carbon dioxide in weathering of rocks is an effect of erosion, the familiar process which tends to reduce heights of land to a low slope, declining to sea level. This tendency is opposed by those internal forces of the earth's mass which depress sea bottoms and relatively uplift continents and mountain ranges. The persistent attacks of the sun's energy are directed against.the earthworks raised by terrestrial forces. It is the fabled fight of the powers of light and air against the powers of the dark underworld; and the former never pause, whereas the latter sleep for ages, and, awaking, exert themselves
