SECTION III.--CHANGES OF ELEVATION AND SUBSIDENCE, 73 SECTION IV.--CHANGES ON THE SURFACE OF THE EARTH, 85 SECTION V.--CHANGES OF CLIMATE, 88 SECTION VI.--ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES, 91 CHAPTER IV. OF THE CAUSES OF GEOLOGICAL PHENOMENA. SECTION I.--ATMOSPHERIC CAUSES, 95 SECTION II.--CHEMICAL ACTION, 97 SECTION III.--ORGANIC CAUSES, 101 SECTION IV.--AQUEOUS CAUSES, 103 SECTION V.--AQUEO-GLACIAL ACTION, 120 SECTION VI.--IGNEOUS CAUSES, 127 CHAPTER I. OF THE MATERIALS WHICH COMPOSE THE CRUST OF THE EARTH. SECTION I.--ELEMENTARY SUBSTANCES. There are about sixty substances known to the chemist which are considered as elementary; but most of them are rarely met with, and only in minute quantities. A few of them are, however, so abundant, in the composition of the crust of the earth, as to render some attention to them necessary. _Oxygen_ is more widely diffused than any other substance. It is an ingredient of water and of the atmosphere, the former containing eighty-eight per cent., and the latter twenty-one. Nearly all rocks contain oxygen in combination with the metallic and metalloid bases, and the average proportion of oxygen which they contain is about forty-five per cent.; so that it will not differ much from the truth to consider the oxygen in the earth's crust as equal in weight to all the other substances which enter into its composition. _Hydrogen_ occurs in nature principally in combination with oxygen, forming water. It is also an ingredient in bitumen and bituminous coal. _Nitrogen_ is confined almost entirely to the atmosphere, of which it forms four-fifths. It enters into the composition of some varieties of coal, and is sparingly diffused in most fossiliferous rocks. One of the most important substances in nature is _carbon_. It constitutes the principal part of all the varieties of coal, as well as of graphite, peat and bituminous matter. A much larger amount of carbon exists in the carbonic acid which is combined with the oxides of the metalloids and metals. The most abundant of these compounds is limestone, which contains about twelve per cent, of carbon. In the neighborhood of volcanoes _sulphur_ is found pure and in a crystalline form. It is a constant ingredient in volcanic rocks, and in several of the most important ores, particularly those of lead, copper and iron. The most abundant sulphate is gypsum, which contains twenty-six per cent, of sulphur. In small quantities it is widely diffused in rocks, and in the waters of the ocean.