A little above Brandeis it picks up the Iser, which, like itself, comes down from the Riesengebirge, and at Melnik it has its stream more than doubled in volume by the Moldau, a river which winds northwards through the heart of Bohemia in a sinuous, trough-like channel carved through the plateaux.
Its waters are mainly collected from the rainfall upon the plateaux, and from the hot springs and geysers, most of which are within its drainage area.
But we have the same phenomenon in single varieties of man, such as the American, which inhabits alike the frozen wastes of Hudson's Bay and Tierra del Fuego, and the hottest regions of the tropics, - the low equatorial valleys and the lofty plateaux of the Andes.
There are numerous large expanses of level country, the most notable of these being the plains (cameos) of the Tagus valley, and of Aviz or Benavilla, Beja and Ourique, in Alemtejo; the high plateaux (cimas) of Mogadouro in Traz-os-Montes and Ourem between the Tagus and the upper Sorraia; the highly cultivated lowlands (veigas) of Chaves and Valenta do Minho in the extreme north; and the marshy flats (baixas) along the coast of Alemtejo and the southern shore of the lower Tagus.
They come out as mica-schists under the Carboniferous sandstones of northern Antrim, and disappear southward under the basaltic plateaux.