Swan River, on the western side of the continent, has much the same character; and from that river to the Gulf of Carpentaria, along the whole of the western and northern shores of the continent, there are none. The want of water makes it hardly possible to explore the interior of this continent. No country stands more in need of a complete system of irrigation, which could easily be accomplished from the nature of the rivers, which lie in deep channels, and might be converted into canals by dams, from whence the water might be conveyed by channels over the surrounding country, as in Lombardy. CHAPTER XX. Lakes—Northern System of the Great Continent—Mountain System of the same—American Lakes. THE hollows formed on the surface of the earth by the ground sinking or rising, earthquakes, streams of lava, craters of extinct volcanos, the intersection of strata, and those that occur along the edges of the different formations, are generally filled with water, and constitute systems of lakes, some salt and some fresh. Many of the former may be remnants of an ancient ocean left in the depressions of its beds during its retreat as the continent arose. Almost all lakes are fed by springs in their beds, and they are occasionally the sources of the largest rivers. Some have neither tributaries nor outlets; the greater number have both. The quantity of water in lakes varies with the seasons everywhere, especially from the melting snow on mountain-chains and in high latitudes, and from periodical rains, between the tropics. Small lakes occur in mountain-passes, formed by water which runs into them from the commanding peaks; they are frequently, as in the Alps, very transparent, of a bright green or azure hue. Large lakes are common on table-lands, and in the valleys of mountainous countries, but the largest are on extensive plains. The basin of a lake comprehends all the land drained by it; consequently it is bounded by an imaginary line passing through the sources of all the waters that fall into it.