Causes and Consequences of Configuration.

54. It will be seen from the preceding paragraphs that the three great elements in the configuration of the land are relative position, contour, and vertical relief. By relative position is meant the place an island or continent occupies on the surface of the globe; and just as this position is frigid, temperate, or tropical—as it lies in broad parallelism along the same zone, or crosses meridionally two or more zones-so will its climate be genial or ungenial, uniform or diversified. The causes that have determined the position of the existing continents lie far, as yet, beyond the indications of science, though we may rest assured that, in the gradual shiftings of volcanic energy from centre to centre, and in the alternate uprise and submergence of large tracts of the earth's crust, we behold the results of a definite and continuously operating law. By contour is meant the outline or figure which the land receives by being surrounded by the water; and just as this outline is simple and uniform, or irregular and indented by seas, bays, and gulfs, so will its external conditions be varied, and its coasts better fitted for the purposes of navigation and commerce. This contour depends mainly, of course, upon the original inequalities of the crust, but partly also upon the waste and degradation which all coast-lines undergo from the ceaseless action of the ocean. Combined, as in the case of Europe, with favourable relative position, it exercises most important influences on climate, productions, industry, enterprise, and civilisation. By vertical relief is meant the elevation of the land above the level of the ocean: and just as this is regular or irregular, low or lofty, so will the whole character of a country be determined. A few hundred feet more or less of elevation is sufficient to change the whole physical aspects of a country-converting arable fields and vineyards into pasture-lands, pasture-lands into pine-forests, and pineforests into regions of everlasting snow and glacier. This vertical relief of continents and islands depends, as already noticed, mainly on their lithological or rocky structure, on the intensity and continuance of the vulcanic forces to which they have been subjected, and on the amount of waste and degradation they may have subsequently undergone. Thus, during the slow oscillations of the earth's crust, there are few tracts of the existing land that have not been more than once under the waters of the ocean; hence the amount of denuding, smoothing, and rounding of their reliefs. There are also few tracts that have not been subjected to a recurrence of volcanic activity; and hence, in proportion, the

greater irregularity, abruptness, and variety of their surfaces. And even after elevation into permanent islands and continents, these surfaces are all less or more subjected to aqueous and atmospheric waste, which is slowly but irresistibly moulding them into newer outlines, according to the harder or softer nature of the rock materials that compose them. The relief of the land is thus a thing of slow but incessant fluctuation, affected by every uprise and depression of the earth's crust, by every volcanic outburst, and by every action of the aqueous and atmospheric forces that waste and wear down the exposed surfaces. So gradually, however, do these forces operate, that no appreciable change is produced for centuries; and thus we are led to associate ideas of permanency with what in reality is incessantly but infinitesimally changing.

NOTE, RECAPITULATORY AND EXPLANATORY.

The facts connected with the general configuration of the land have been so fully stated in the preceding chapter, that very little additional explanation is required. Relative position, contour, and relief, are alike influential in modifying the external conditions of a region; hence the importance of their accuracy in all geographical descriptions. Latitude and longitude determine the position; a sufficient number of similar observations fix the contour; and trigonometrical survey or barometrical observation (mercurial or aneroid) can define the relief or outline of elevation. Position and contour are readily shown on maps by actual outline; relief can be indicated by a scale of shading, or more accurately by a system of contouring, as shown in Plate I. (Chartography) of the 'Physical Atlas.' Thus, taking the shore-line, which is all on the same level, as the first contour, we may have similar lines taken at every 50 feet or every 100 feet of ascent, and these (when the observations are sufficiently numerous) will exhibit, with great accuracy, the risings and fallings of the surface configuration. Such a series of lines gives, as it were, the moulding or model of the surface, and a section of this model, in any direction, will show the profile for that direction. Suppose an island, for example, with its contours taken at every 100 feet of ascent, to be represented by fig. 1; then fig. 2 will show the same in profile or absolute elevation in the line of section A B. The closer the lines of contour, the more rapid or steeper the

ascent, and the more widely apart they are, the flatter is the slope of uprise. It is by such sections that the profiles of the different

continents are usually shown by geographers, as will be more fully illustrated when we come to treat of the mountains and tablelands in the succeeding chapters.

Should the student desire to enter more fully into the subject of position, contour, and relief, with their ultimate bearings on the features and characters of the respective continents, he will find more ample details in Guyot's 'Earth and Man;' in Mrs Somerville's Physical Geography;' and in several of the chapters of Humboldt's 'Cosmos.'

VI.

THE LAND-ITS HIGHLANDS.

Mountains and Mountain-Systems.

55. HAVING directed attention to the general configuration of the land as dependent on its contour and relief, we now proceed to describe the special features of that relief as exhibited in its mountains and mountain-chains, its table-lands and plateaux. And, first, of the mountains, which have long attracted the atten tion of geographers, as one of the most obvious causes of climatic, vital, and political diversity. Defining a mountain as "any portion of the earth's crust rising considerably above the surrounding surface," it must be remembered that mountain-ranges are not mere ridges, rising abruptly on one side and descending as abruptly on the other, but are in reality elevated tracts, often of great breadth as well as length, and consisting of rounded heights, lofty peaks, and boldly escarped plateaux, with intervening valleys. As, geographically speaking, a mountain-range is not a single ridge of elevation, but a brotherhood of many elevations, so, geologically speaking, it is not a simple upheaval, the result of one paroxysmal outburst, but the work of innumerable volcanoes and earthquakes operating through untold ages. A mountain-chain ten or twenty thousand feet in height and hundreds of miles in length, is thus a thing of slow and gradual growth-upheaval after upheaval, and eruption after eruption; now with widespread intensity, now slumbering for generations, and again with renewed activity, each contributing to augment the mass of final accumulation. And as the vulcanic forces necessarily upheave and disrupt the stratified systems through which they pass, bearing them up on the mountain-flanks, or imbedding them amongst their ejectments, so we can judge of the relative ages of mountain-ranges from their associated strata, and say whether they were formed during primary, secondary, or tertiary eras. It is in this manner

that Geology enables Geography to account for the external characters of mountains, and to explain why some should be massive and rounded in outline, others serrated, with splintery peaks and pinnacles-some conical and dome-shaped, and others, again, terraced by crags, or sloping away in long, gentle declivities.

56. Understanding the general character and formation of mountainous elevations, we may explain that the term mountain is usually applied to heights of more than 2000 feet-all beneath that height being regarded as hills, and those of still minor elevation as hillocks. A mountain-chain or mountain-range is a series of elevations having their bases in contact, and their axes or lines of elevation continuous, over a considerable extent of countryas the Grampians, Urals, or Andes; a group consists of several ranges more or less connected; and a system, of several groups that evidently belong to the same set of geologic operations. Mountain-summits are distinguished by such terms as cones, when gradually tapering to a point, as in volcanoes; domes, when more massive and rounded; peaks, when abrupt and insulated; and by the French word aiguilles, or needles, when still more pointed, splintery, and detached. The terms undulating, serrated, rugged, and the like, applied to their outlines or profiles, are so familiar in everyday language, as to require no special explanation. Mountainsides consist of slopes, terraces, escarpments, and precipices, or of a combination of these; and these outlines depend chiefly on their geological structure, and partly on the amount of waste and degradation to which they have been subjected. In general, mountain-ridges have a long, gentle declivity on one side, and a short and abrupt one on the other; and where this occurs, the longer declivity is spoken of as the slope, and the shorter one as the counter-slope. This slope and counter-slope is equivalent to what is known in Britain as "crag and tail,"—most of our isolated hills presenting a bold precipitous front to the west or north-west, and a long slope or tail to the east or south-east-a configuration the obvious result of denuding currents, as the land gradually uprose from the waters during the pleistocene epoch. The depressions and narrow valleys which occasionally intersect mountain-chains are known as defiles and passes; and these, offering, as they often do, the only means of transit across mountain-barriers, have ever been objects of political and commercial importance. Some of the most celebrated deeds recorded in history are associated with these passes, and modern engineering is still in search of their easy gradients and facilities.

[We have said that the outlines of mountains depend chiefly on their geological structure, and partly, also, on the amount of waste and degra