sandstone are often varied and picturesque, and in general its slopes are dry and of moderate fertility.

9th. The Silurian System (so called from its typical development in that district of Wales anciently inhabited by the Silures) consists of numerous slaty or hard shaly beds, with sandstones, conglomerates, and intercalated limestones. As in all the older and deeper-seated rocks, there is often a tendency to crystalline structure among its strata, and these are not unfrequently traversed by metalliferous veins-tin, copper, silver, and gold. Its fossils are eminently marine, and consist almost wholly of the lower or invertebrate orders (corals, shellfish, and crustacea), few fishes being found in its strata, and these only in the upper portions of the system. Flanking and often borne up by the older granitic hills, the physical features of the formation are frequently irregular and mountainous, but, from the softer nature of the rocks, are more rounded and massive, and less abrupt and precipitous, than those of the crystalline or metamorphic system.

10th. The Metamorphic System (so called because its strata have undergone a metamorphism or change by heat, pressure, and chemical change, from ordinary sandstones, shales, and limestones into hard crystalline schists), consists of such rocks as gneiss, quartz-rock, mica-schist, clay-slate, and primitive marbles. They yield no fossils; but whether all traces of life may have been obliterated by the mineral change these rocks may have undergone, or whether they were deposited before life existed on the globe, Geology cannot determine, and merely expresses the fact by the term Hypozoic, which signifies lying beneath the strata that are decidedly fossiliferous. These crystalline schists are generally found at high angles, flanking or composing the main mass of the older mountains; and from their hard splintery nature present those peaks and ridges that confer on primitive districts their bold, wild, and alpine character.

32. Such are the leading features of the stratified systems, and by such, with a little field-practice, they may be readily detected in the various areas they occupy. As with the stratified so with the unstratified, each great group has its own peculiar features, and though perhaps less sharply defined, they are still sufficiently distinct to be recognised in hill and mountain ranges as volcanic, trappean, and granitic :

1st. The Volcanic, as already stated, are generally associated with the more recent formations, and consist of trachytes, basaltic lavas, vesicular lavas, scoriæ, and other similar products-loose and less consolidated in the more recent and active, and harder and more compact in the older and extinct volcanoes. They rise

up in dry rocky hills, more or less conical and crateriform; and these are, perhaps, more frequently grouped round some common centre than arranged in linear or axial directions.

2d. The Trappean, so called from the terraciform aspect of many of the hills they compose (Swedish, trappa, a stair), consist of greenstones or whinstones, basalts, felstones, porphyries, tufas, and other kindred rocks, and are generally associated with the secondary and upper primary strata. They are usually arranged in hill-ranges more or less persistent, and from their higher antiquity and longer subjection to wasting influences, are more worn into rounded heights, exposed crags, slopes, and terraces, which confer on the landscape a beauty and diversity peculiarly their Their soils being dry and genial, the trap hills of a country are generally possessed of great amenity and fertility, and constitute, perhaps, the most valuable agricultural portions of the district in which they occur.

own.

3d. The Granitic, or oldest series of igneous rocks, consists of granites, syenites, porphyries, serpentines, and the like, which, from their more ancient and deep-seated relations, are generally hard and crystalline in texture and massive in structure. They constitute the nucleus or backbone, as it were, of all the higher and older mountain-chains-elevating the metamorphic schists into splintery peaks and abrupt ridges, or presenting of themselves broad massive shoulders of cold sterile moorland and unprofitable heath.

Connection between Geology and Physical Geography.

33. Understanding the nature of the preceding subdivisions, the student will be able to attach some intelligible idea to such phrases as the river "cutting its way through secondary strata ;" "lying in an irregular tertiary basin;" "crossing a plateau of triassic sandstones;" 99 66 a cold retentive soil derived from the subjacent carboniferous rocks;"" intersected by a low range of trap hills, whose grassy slopes and terraces;" "bounded by sterile granitic ridges, whose snow-clad summits;" and hundreds of others that are of continual occurrence in geographical descriptions. He will still more fully perceive the intimate connection of geology with his own immediate science when he reflects that many of the soils which give character and colouring to vegetable growth are derived directly from the disintegration of the subjacent rocks, and are further affected in their fertility by the porous or retentive nature of the beds on which they rest. The

soil derived from the Chalk, for instance, is light and absorbenthence the short sweet herbage of the "downs" of the south of England; that derived from the marls of the Trias is stiff, retentive, and less fitted for tillage than for pasture; the disintegration of trap rocks, rich in soda, potash, and lime, yields a fertile genial soil, which their structural fissures ever keep dry and pulverulent; while the scanty disintegration of an impervious granite is poor, cold, and barren. We have seen, moreover, that every geological formation is less or more characterised by its own special features. The soft rounded outlines of a chalk range is altogether distinct from the slopes, terraces, and conical heights of secondary trap hills; while these, on the other hand, are widely different, both in outline and in vegetable covering, from broad-shouldered mountains of granite, or the splintery peaks of metamorphic strata. In fine, these formations and rock-groups constitute the framework on which the geographical features are moulded; and their inherent characters, as subject to external and internal change, have produced, and are ever producing, new variety of superficial aspect. Every hill that volcanic energy raises above the general surface, every inequality produced by earthquake convulsion, every glen eroded by river-action, every plain formed by the deposition of alluvial silt, and every inch that one region is elevated above the ocean, or that another may subside beneath it, disturbs the existing geographical equilibrium, confers new features on the landscape, and modifies the habitats of vegetable and animal existences. Geology and geography are inseparably connected; and thus it is that some acquaintance with the nature and sequence of the rock-formations that constitute the solid crust, and with the causes that produce them, becomes so indispensable to the student of physical geography.

NOTE, RECAPITULATORY AND EXPLANATORY.

In the preceding chapter we have directed attention to the rocky structure of the globe as that which gives colour and character to all external phenomena, and is therefore of paramount importance to the student of physical geography. We have spoken of the "crust" composed of materials that can be seen and investigated as distinct from the "interior," of which nothing can be known by direct observation. This crust, composed of stratified and unstratified, or of water-formed and fire-formed

rocks, is held in habitable equilibrium between the disintegrating forces of water from without, and the reconstructing forces of fire from within. To these two opposing powers are chiefly owing the continuous geological modifications of the earth's crust -each modification representing a period during which certain rocks were formed, and the remains of plants and animals that then lived entombed in them in greater or less perfection. These formations being arranged by the geologist in chronological sequence, and each varying in mineral character, and consequently conferring on the landscape different aspects, it is of importance in physical geography to know the order of this arrangement, and the nature of these distinctions. These formations, we stated, had been arranged into certain systems, and these systems again grouped into certain periods, according to the leading features of their fossils, thus :

As with the stratified so with the unstratified-their arrangement into Volcanic, Trappean, and Granitic, expressing the relative antiquities and nature of the great igneous groups that give character and individuality to the hills and mountain-ranges of the globe. So far, then, as diversity of surface, fertility of soil, character of landscape, and similar peculiarities, are concerned, the connection between physical geography and geology is intimate and inseparable; and hence the necessity that the student of the one should be less or more acquainted with the leading facts of the other. It by no means follows, however, that the student of geography should be deeply read in the theoretical problems of geology. An acquaintance with its general principles, and the causes that are incessantly productive of new modifications of the earth's crust, the leading formations that compose that crust, the nature of their rocks, and the character which these impart to the soil and landscape, is all, or nearly all, that he requires; and this information he can readily obtain from such manuals of the science as the Author's 'Introductory and Advanced Text-Books,' or Sir Charles Lyell's 'Principles' and 'Manual,' should he wish to extend his information beyond the outline sketched in the foregoing chapter.

IV.

DISTRIBUTION OF LAND AND WATER.

Their Relative Positions and Areas.

34. HAVING noticed the general conditions of the earth as a member of the solar system, and having also adverted to the structure of its own rocky crust as the groundwork of all geographical diversity, we now proceed to consider the more intimate relations of its surface, as composed of Land and Water. When we speak of the earth's surface as composed of land and water, we mean that all the more elevated portions of the crust stand out as dry land, while the lower and more depressed are covered by the waters of the ocean. Geologically speaking, the relations of land and water are continually, though slowly, undergoing change and modification; geographically speaking, and within the limits of such mutation, what are the respective areas of land and water, their positions on the earth's surface, their actions and reactions on each other, in consequence of their different extents, positions, capacities for heat, &c., and what the influence of these reactions on currents, winds, rains, and all those kindred phenomena that constitute climatic diversity? The present distribution of land and water tends to the production of certain physical and vital results, and the explication of these results forms the sum and substance of our science; but no portion of the present distribution could be altered either in its position, extent, or configuration, without being accompanied by a modification of these results, so intimately is the one element bound up with the other in the production of a definite and harmonious whole.

35. As formerly stated, the superficial area of the globe has been computed at 197,000,000 square miles. Of this amount about 51,500,000 consists of dry land, and about 145,500,000 of water; or reckoning the whole surface of the globe as equal to 1000 parts, 262 of these parts will consist of land and 738 of water. The