This wealth of plant life is confined to the littoral and the coastal valleys, but the central valleys and the plateaux have, if not a varied flora, a considerable wealth of timber trees in every way superior to the flora inland in the same latitudes.
When the warmer interglacial periods recurred the polar and continental ice-caps melted and the sea-level became raised again - that is, there was submergence of the eroded plateaux formed as indicated above.
Devonian beds also lie upon the older rocks in the Matto Grosso and other provinces in the interior of Brazil, where they generally form plateaux of nearly horizontal strata.
Neveu-Lemaire, Les Lacs des hauts plateaux de l'Amerique du Sud (Paris, 1906); M.
(3) In the steppe the vegetation is that which prevails in similar soil from Central Asia to Algeria; but many of the arborescent plants that grow in the rockier and more irregular plateaux of western Asia, and especially of Persia, have been reported as missing.
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.
More than double, to the southern, the difference being mainly due to the zone of plateaux and sierras.
The mean elevation, estimated by Elie de Beaumont at 1500 metres (4900 ft.), has been sensibly diminished by the addition of that zone to the system, and it must now be placed at only 1200 metres (3930 ft.) for the range as a whole; so important a part is played by the above-mentioned plateaux of small elevation in a chain whose highest summit reaches 11,168 ft., while the passes show a greater altitude than those of the Alps.
A special characteristic of the Yemen highlands is that fields and inhabited sites are found at the highest elevations, the mountain-tops forming extensive plateaux, often scarped on every side and only accessible by difficult paths cut in the cliffs which encircle them like the escarpments of a natural fortress; a remarkable example of this is Jebel Jihaf on the Aden border, 8000 ft.