where the lofty Nejd Plateau, with its rain-gathering peaks
over five thousand feet high, varies its wide pastures with well tilled
valleys abounding in grain fields and date-palm groves.[1121] Along the
whole Saharan slope of the Atlas piedmont a series of parallel wadis
and, farther out in the desert, a zone of artesian wells, sunk to the
underground bed of hidden drainage streams from the same range, form
oases which are the seat of permanent agriculture and more or less
settled populations. The Saharan highlands of Tibesti, whose mountains
rise to 8,300 feet, condense a little rain and permit the Tibbus to
raise some grain and dates in the narrow valleys.[1122]

[Sidenote: Irrigation and horticulture.]

The few and limited spots where the desert or steppe affords water for
cultivation require artificial irrigation, the importation of plants,
and careful tillage, to make the limited area support even a small
social group. Hence they could have been utilized by man only after he
had made considerable progress in civilization.[1123] Oasis agriculture is
predominantly intensive. Gardens and orchards tend to prevail over field
tillage. The restricted soil and water must be forced to yield their
utmost. While on the rainy or northern slope of the Atlas in Algiers and
Tunis farms abound, on the Saharan piedmont are chiefly plantations of
vegetables, orchards and palm groves.[1124] In Fezzan at the oasis of
Ghat, Barth found kitchen gardens of considerable extent, large palm
groves, but limited fields of grain, all raised by irrigation; and in
the flat hollow basin forming the oasis of Murzuk, he found also fig and
peach trees, vegetables, besides fields of wheat and barley cultivated
with much labor.[1125] In northern Fezzan, where the mountains back of
Tripoli provide a supply of water, saffron and olive trees are the
staple articles of tillage. The slopes are terraced and irrigated, laid
out in orchards of figs, pomegranates, almonds and grapes, while fields
of wheat and barley border the lower courses of the wadis.[1126] In the
"cup oases" or depressions of the Sahara, the village is always built on
the slope, because the alluvial soil in the basin is too precious to be
used for house sites.[1127]

[Sidenote: Effect of diminishing water supply.]

The water supply in deserts and steppes, on which permanent agriculture
depends, is so scant that even a slight diminution causes the area of
tillage to shrink. Here a fluct