The Mesozoic or Secondary strata (Triassic, Jurassic, and Cretace ous) are less valuable on the whole with regard to the economic minerals which they contain, but more valuable in respect of the soils into which they weather. In the Triassic rocks, coal and salt are found in different parts of the world. The metamorphosed Jurassic rocks of California are gold-bearing, while the unaltered Jurassic formations, outside of North America, contain more coal than any other formation except the Carboniferous. In North America, on the other hand, the Cretaceous regions of the west are the most productive in coal, and contain large supplies of a lignitic character. Iron is found in the Jurassic oolites of England and Europe.
The Cainozoic era is divided into the Tertiary and Quaternary periods. During the first of these, important changes took place in the form of the land and the great mountain ranges of the globe were upraised. Volcanic outpourings on a large scale also occurred in different parts of the world at this time. The mineral wealth of the sedimentary rocks formed during the Tertiary period is not very great, but coal is found in those of Washington and Alaska, in parts of Europe, and in Japan. Oil occurs in similar formations in Europe and North America, and amber in North Germany. The Quaternary period is of most importance in relation to the influence upon soil of the ice-sheet which extended over considerable areas of Europe and North America. This will be discussed later.
The above account of the distribution of economic minerals in rocks of different geological periods must not be considered as exhaustive, and numerous other instances will be referred to in the course of the present work. It is essential, however, that the student should from the beginning realise the importance of the geological factor in economic geography, and it is for this reason that these illustrations have been given.
The physical and chemical properties of the soil vary according to the composition of the rocks from which they are derived, and these variations affect its fertility and suitability for vegetation. On the whole, crystalline rocks such as gneiss do not provide a suitable environment for plant life. The soils derived from them are often thin, as they weather slowly, and they are usually wanting in lime and other constituents of fertility. Granites, also, though rich in phosphates, are often poor in lime, and do not, as a rule, form a fertile soil. On the other hand, some eruptive rocks are very productive, as they may contain both lime and phosphates, and are at the same time retentive of moisture. Basalt, for example, often weathers freely, and responds readily to good cultivation. Of the soils formed from sedimentary rocks those derived from limestone are generally fertile, as potash is in many cases present in addition to lime. That " a limestone country is a rich country " may be illustrated by reference to the blue-grass region of Kentucky ; but in some districts, more especially in upland regions, the soil is very thin and cultivation is impossible. Sandstone soils vary greatly in character. Much
depends upon the nature of the cementing material which holds the grains of sandstone together. If it happens to be lime, the soil may be fertile, but, if lime is absent and the cementing material is siliceous, the sandstone will disintegrate into a poor and infertile soil. On the Bunter Sandstone of Germany a forest vegetation alone is possible, while some of the richest soils of Great Britain are upon the Old Red Sandstone. The intermixture of the debris of different kinds of rock frequently leads to a soil of great fertility. Thus the alluvial soils deposited by rivers on their flood plains, and at their deltas, are often among the most productive. Many glacial soils are fertile for the same reason, though it must not be assumed that all are so. Much depends upon the source from which the rock waste comes, and the conditions under which it is deposited.
A knowledge of the morphological processes determining the physical evolution of a region frequently throws much light upon its economic development. The folded mountain ranges of the world are generally higher than its dissected plateaus, their geolog ical structure and river systems are different, and they exercise an influence peculiarly their own upon human progress. Plains of accumulation, again, are not the same as plains of denudation. In the one the strata are generally weak and unconsolidated, and minerals are usually, though not always, wanting ; in the other the rocks are hard and consolidated, and great mineral wealth may exist. The plain of Western Siberia is an example of the first type, and that of Central Russia of the second. The value of rivers to man varies with the stage of development at which they have arrived. A river in early youth, descending from a mountain range, is generally useless for navigation, though it may be pro ductive of much water-power. On the other hand, a river which has reached maturity, such as the Lower Mississippi, is navigable, but is without surplus energy. A transverse valley again tends to be narrow and to have steep sides, while a longitudinal valley is broad and suitable for settlement. Changes in the relative level of land and sea have had important economic results. When the land has sunk relatively to the sea, river mouths have often been drowned, and good harbours formed, as was the case along the coast of New England. Further south, along the Atlantic seaboard, the land has risen, and good harbours are few and fax between. The processes which led to the formation of the continental shelf, upon which the British Isles stand, account for the fishing industry of these islands, and for the high tides which have played so important a part in the development of their ports.