opened out by volcanic action, or cut through the soft soil by the constant efforts of the streams by which it is traversed; such valleys of excavation as these have been sometimes not unaptly called negative valleys. 3. Rivers. Scarcely less important than the study of mountains is that of the effect of rivers in modifying the geographical configuration of a country. From their sources in the mountain recesses to their final disemboguing in the ocean, their course, their currents, and their shores afford an endless variety of remarks and observations. The depth and colour of the water, the rate at which it flows, the cataracts it forms with the rocks over which its waters are precipitated, the eddies and currents by which its course is marked, are all deserving of notice, as are also the rocks and shoals which obstruct its uniform progress, either interfering with its navigation, or, by projecting beyond its ordinary banks, throwing back the rushing torrent on the opposite shores, as has been so graphically described by the Latin poet :— Vidimus flavum Tiberim, retortis thus causing the gradual fall of the cliffs by undermining their precarious foundations. Nor in noting the size or extent of rivers should we neglect to state how far they are navigable, for what vessels, and by what means; whether the mouth is constantly free, or whether closed by a bar, and how much water there generally is over it, and how far up the tides usually extend. Some rivers, however, are not only closed by a bar, but, as in the case of Western Australia, are, during periods when the water is low, completely masked by the sand-hills or dunes which are blown up, forming a continuous bank with the hills which skirt the shores, and only when freshets of more than ordinary force come down are these sandy barriers overthrown, and the rivers enabled to find an uninterrupted outlet. In other cases the effect of beaches thrown up by the constant set of currents in one direction is not so absolutely insurmountable, the streams are only partially deflected from their proper course, and, instead of flowing into the sea in a continued line, are compelled to run for some distance parallel to the coast, until the accumulated backwater has acquired sufficient power to overcome the diminished resistance of the sea-beach: this, however, more properly belongs to the consideration of the coast-line. But the description of a river will be imperfect, unless we also state the number and character of the streams which fall into it. And here we have to consider the angle at which the rivers join each other, whether the direction of the main stream is altered or not by the junction, the relative size of two confluent streams, and which of them may be said to preserve its former course with the smallest deviation. On the true description of these details must depend the question as to which of two confluent rivers should be considered as the main or parent stream. Rivers are said to be confluent when both branches are nearly equally deflected from their former direction, and the united streams may be said to be the resultant of two contrary forces. An affluent is a stream which falls into another, called the recipient, without changing the direction of the latter, and entirely losing its own. d b a and b are confluent streams, d is an affluent falling into c, the recipient. An affluent, too, will generally be found to be smaller than its recipient, and may often be more correctly called a rivulet or a torrent; and here it may be remarked that there is great advantage in attending to the true and proper use of these relative terms, rivers, torrents, rivulets, or brooks, the two latter being more or less synonymous, and a torrent being generally applied to a rapid mountain stream; all these, more or less, bring down from the hills detritus, which is deposited at the mouths of the streams, or wherever other natural causes retard the rapid flow of water. In these cases deltas are formed, which deserve examination, and are either fluviatile, lacustrine, or marine, according as the river empties itself into another river, a lake, or the sea. * But there are other important characters which deserve attention in the description of a river; and chiefly the name s to be considered. Does it change during its course, and where and when? How far up from the mouth is the same name preserved? and is it the same on both banks? What is its origin, and by whom was it first given? Then we must inquire what islands are met with in its course? Where are they situated? Are they low? subject to inundation? marshy or rocky? or do they stand high above the level of the stream? Are they cultivated or not? What are their natural productions? By what animals are they inhabited? Again, is the river at all affected by rapids, or shoals, or cataracts? and what are the peculiar characteristics of these impediments to navigation? Does the tide flow in them, and how far up is it felt? Does the river abound with eddies or whirlpools, and how are they occasioned? Do they interfere with navigation or not? Are they accompanied by rocks or shoals? Again, we must ascertain what fords a river offers, and what depth of water is generally found over them: the nature of the bed of the river, particularly in the case of a ford, should also be carefully ascertained. In addition to these remarks, many other important * See Col. J. J. Jackson's work, 'What to Observe.' London, 12mo., 1841. peculiarities will often occur to the careful observer. In some countries, particularly in secondary limestone districts, the rivers are remarkable for their subterranean courses. Suddenly emerging in large volumes from the bases of lofty mountains, they flow across rich alluvial plains, and are then as suddenly lost in the cavities of another mountain, again to issue forth to the light of day in a distant region, after their subterranean courses. Nor should the traveller omit to notice, when crossing a river, the direction in which it flows as regards his own route, whether to the right or to the left. Instances are not wanting of distinguished travellers having been unable to connect their observations from not having sufficiently attended to this point. 4. Springs.-The phenomena connected with the outbursts of water from the surface of the earth are not only of the greatest interest, but a correct observation of them will be attended with much practical advantage. The traveller should state, approximatively at least, their size or volume, and the nature of the rock or soil out of which they rise; also whether they are pure or mineral, and what deposits are formed about the orifices through which they issue; how they are affected by different seasons; whether their flow of water is constant or intermittent, like the famous spring described by Pliny on the shores of the Lake of Como; whether they are of ordinary temperature or thermal; and if the latter, it is desirable to ascertain the degree of heat by means of a thermometer: the touch alone is a very vague and uncertain test. Let him endeavour also, when it can be done conveniently, to procure specimens, in closely sealed bottles, of the water of such springs as appear to possess mineral properties, or to contain salts in solution, for the purpose of analysis at home. Naval officers whose ships are at hand have in this respect great advantages over *Styria and the neighbourhood of Trieste. The loss of the river Mole in the chalk of the North Downs in Surrey is an instance of this phenomenon on a small scale within a distance of 25 miles from London. those whose only mode of transport is on horseback or on camels. 5. Lakes. These sheets of water, varying greatly in size, form very important features in the geographical description of a country, and the traveller should carefully remark their connexion with the other hydrographical characters of the district. Whether they constitute the sources of rivers, or are their ultimate recipients; whether they are or are not connected with the ocean or other great seas; their levels with regard to the ocean, particularly when at a lower level; what rivers, if any, flow into or out of them, and whether they contain fresh water or salt. The following remarks from Colonel Jackson's work, already quoted, are very appropriate :-" With regard to lakes in general, the observations to be made upon them may be comprehended under the following heads : "Name; geographical and topographical situation; height above the level of the sea, and as compared to other neighbouring lakes; subterranean communication; form, length, breadth, circumference, surface, and depth; the nature of the bed and of the borders; the transparency, colour, temperature, and quality of the water; the affluent streams and springs; the outlets, the currents; the climate, soil, and vegetation of the basins; the height and nature of the surrounding hills when there are any; the prevailing winds; the mean ratio of evaporation compared with the quantity of water supplied, and any particular phenomena; the navigation and fisheries of the lake; formation and desiccation of lakes." This latter point, depending as it often does on the relative elevation or subsidence of the country, belongs also to the kindred science of geology. Connected with the question of lakes are the scarcely less important features of lagoons and marshes, and smaller hollows or ponds; the extent of these should be ascertained, as well as whether they are connected with the sea or not, and what portions of them become dry and passable during the summer or at other periods of the year. Peat bogs, in many cases the remains of former lakes, may also be classed |