PHYSICAL CONDITIONS OF ECONOMIC ACTIVITY Geography may be defined as the study of the influence exerted upon the economic activities of man by his physical en vironment, and more especially by the form and structure of the surface of the land, the climatic conditions which prevail upon it, and the place relations in which its different regions stand to one another. These physical factors, it is true, do not determine absolutely the character of economic life, but they exercise a control over it which is more apparent, no doubt, in the earlier stages of human history, but which is no less real in advanced civilisations when man has learned to respond to his environment and to obtain from it an increased benefit.
In order to pursue a study of the character here indicated, it is necessary to have recourse to much information derived from other sciences. An appeal must be made to the geologist and geomorphologist for many facts regarding the structure and forma tion of the surface of the earth ; from the metallurgist and the mining engineer must be obtained some knowledge of the value of minerals and fuels accessible to man ; the general principles determining climate must be accepted from the meteorologist ; while the botanist and the agricultural chemist must supply the necessary information regarding plant life. To the economic geographer belongs the task of correlating these different facts and estimating their influence upon human activity. In the first place he has to show, among other things, how the distribution of soil and minerals is affected by the physical structure of the earth ; how climate varies with position and configuration ; and how vegetation is determined by climatic and edaphic environ ment. Secondly, he has to consider the extent to which man in his economic aspect is controlled by these various factors, and how far he is able to free himself from their control and consciously to adapt himself to his environment.
From what has been said it is obvious that some knowledge is necessary, not only of the actual configuration of the surface of the land, but of the rocks of which it is composed, and even of the morphological processes by which its present form has been deter mined. The minerals which the rocks contain, the soils into which they weather down, and the different types of land form which they constitute, each with its own potentialities for settlement and development, must all be taken into consideration.
Rocks may be classified as igneous, sedimentary, and metamor phic. Igneous rocks have been formed by the cooling and solidi fication of molten matter, and, while some may have formed part of the original surface of the earth, others belong to more recent times, as is testified by their occurrence among the sedimentary strata. The latter have been formed by the deposition or pre
cipitation of matter derived from pre-existing rocks ; they include conglomerates, sandstones, limestones, and shales. Metamorphic rocks are derived from igneous or sedimentary rocks, which, owing to great pressure, heat, or other causes, have entirely lost their original characteristics. Marble, for example, is limestone which has been metamorphosed by heat.
The oldest known rocks are called Archaean, a term which, although it is now being restricted to the earliest formations, has hitherto been applied to all of pre-Cambrian times. These rocks vary in structure, the oldest consisting of schists and granites, while the more recent are of metamorphosed and in places of unaltered sedimentary material. They weather slowly and frequently have but a scanty covering of soil. On the other hand, they sometimes contain great mineral wealth, as in North America, where the richest iron ores of the continent are found within the Archaean area. In some regions, also, and more especially where there are intrusive igneous rocks, valuable deposits of the precious metals occur.
The Palaeozoic rocks are of sedimentary origin (though they have also been subject in numerous instances to metamorphic action), and contain great deposits of minerals of economic value. In the Cambrian rocks of North America gold is found, while in the lower Silurian strata of the same continent there are large quantities of oil and natural gas, both of which are believed to be due to organic matter included in the rocks at the time of their deposition. Where the metamorphosed Devonian rock of Europe is in contact with intrusive igneous rocks, iron, tin, and copper are frequently found. The Carboniferous period saw the deposition of the great coal measures of the world. In the lower Carboniferous rocks of Scotland and of Russia, coal is found ; but it is in the upper Carboniferous formations that the more important coalfields of Europe and eastern North America occur. Iron is also frequently present in the same formations. The Permian beds in many parts of the world contain large deposits of salt ; the upper Permian, for example, contain the thickest layers of that mineral in Europe, Copper and coal are sometimes also found.