538 TRANSACTIONS OF SECTION E. 
The trend lines of the Mediterranean Atlas, which are well developed in 
Algeria, run from west to east, and seem to disappear gradually toward 
the east before reaching the Tunisian frontier. They do not extend into 
Tunis except in the shape of fault lines and lines of displacement, which 
interfere with the other trend lines, producing a confused topography. 
These regions of Tertiary sandstone have still retained their forests of 
evergreen oaks and cork oaks, while the softer Cretaceous Marls are carved 
into vales covered with rank grass. The course of the Medjerda River 
divides these regions from the division treated of in the next paragraph. 
This river, flowing through a series of fertile alluvial depressions, has 
in a remarkable manner forced its way through the surrounding mountains, 
and, instead of following a continuous longitudinal valley, it drains 
series of fertile elliptical plains, joined by steep gorges cutting through 
the intervening anticlines. Most of these gorges have probably been 
- originated by the capture of the higher lying depressions by the lower 
ones; in some cases it is even probable that the capture was at first 
subterranean, and that some of these canyons have been formed by under- 
ground erosion and by the subsequent subsidence of the roofs of the caves. 
(c) The Central Upland of Tunis. 
In the central district we find a rolling upland, consisting of a series 
of dome-like elevations, running in more or less regular lines and broken 
hy elliptical depressions. These structures have probably been brought 
about by the superposition of the Atlas folding running south-west and 
north-east upon an older system running in meridian ‘directions, and 
which is more visible in the far south. The formation of these domes 
and basins might be illustrated by compressing a sheet of corrugated iron 
in such a manner as to produce a new series of corrugations crossing the 
original ones obliquely. 
(d) Continuous Ridges of the South. 
Toward the south we find elongated upfolds running parallel east and 
west, and dividing Tunis proper from the Sahara. Rich phosphatic deposits 
have been found in the Eocene limestones forming the flanks of these 
anticlines. 
(e) The Margin of the Desert. 
Along the foot of the southern ridges we find a series of alkali flats and 
salt lakes, locally known as ‘ Shotts,’ containing a large amount of chlorides 
of sodium, magnesium, and iodine. At a few points, where fresh water 
is available, are located the flourishing oases of the Djerid. These oases 
have been compared to natural greenhouses irrigated with hot water, the 
temperature of some of the springs reaching over 90° Fahr., giving to 
this group of oases a decided advantage in bringing to early maturity the 
dates, fruits, and vegetables, which are the chief source of wealth of this 
region. 
(f) Coastal Plains (locally called ‘Twnisian Sahel’). 
These extend east of the central uplands to the coast. They are fertile 
lowlands of recent origin, covered with plantations of olive-trees and limited 
toward the east by sand dunes. 
1 There is a striking contrast between the meandering habits and the mature 
character of the river in the longitudinal depressions, half filled up with its own 
alluvions, and its youthful and energetic hehaviour in the transversal canyons, 
