GHARDAIA TO TOUGGOURT. 5 1 



low, flat-topped mountains, which are a feature of the topography about 

 Ghardaia, were soon left behind, and nothing similar was encountered 

 until the vicinity of Ouargla was reached. Between Ghardaia and El 

 Ateuf are small dunes in the valley and sand is drifted along the base 

 of the walls at various places. Near and immediately east of this town 

 the sand is especially abundant and, being shifted by the winds, con- 

 stitutes an ever-present menace to the small gardens belonging to the 

 inhabitants of the place; in order to control its drifting, fences of palm 

 leaves are made or the sand is removed when it becomes too abundant. 

 Often the gardens are abandoned, leaving the palm fences to mark their sites 

 after the sand has gone beyond. Where the air-currents are most powerful 

 or most consistent, or the walls are broken down, the sand may be car- 

 ried in small quantities onto the plain, where it constitutes a mulch, influ- 

 encing in a striking manner the character of the vegetation (fig. 42). 



Finally leaving the valley of the M'Zab about 73 kilometers from Ghar- 

 daia, the trails wound upwards through low rounded hills to the hamada. 

 This is the northern edge of the region of the Gantara, 100 by 1 50 kilometers 

 or more in extent, reaching from the valley of the M'Zab on the north to 

 the region of the dunes to the south. It slopes towards the Oued Igharghar, 

 or the drainage depression connected with this great oued. The Gantara 

 has a few chotts and is crossed by three oueds in the southern portion. 

 It is probably the most arid part of southern Algeria. About 60 kilo- 

 meters of the plain were crossed and here it was gently rolling and stretched 

 without a break to the horizon. The surface resembles that of the hamada 

 at Ghardaia, that is, stones of various sizes, usually small, lie on its surface, 

 but never forming a continuous cover, as in some portions of the Arizona 

 desert. The soil is brown, of fine grain, and with little or no addition of 

 sand. In the innumerable little hollows the soil is deeper than on the 

 slight rounded ridges. Wind is apparently the most potent erosive agent. 



Two chotts were encountered between Ghardaia and Ouargla, one unim- 

 portant, the other large and with many features of interest. The latter, 

 the Chott Mellala, is about 10 by 15 kilometers in size. The trail descends 

 from the plain, winding through a zone of rounded, cone-shaped hills or 

 mamelons, to the floor of the chott, which lies about 60 meters lower than 

 the general level of the plain. The chott was quite dry in November when 

 we visited it, but at rare intervals water is said to flood the central portion. 

 Toward the outer edges the floor is thrown into waves, where the heavy 

 incrustation of salts is broken. In the center the salt crust forms an un- 

 broken and level surface. Gypsum (calcium sulphate) is the predominant 

 salt. On the eastern side a long and high ridge of sand rears its uneven 

 summits. The height of this ridge was estimated to be 250 meters, and 

 was said by Massart to be the largest seen by him in the Sahara. This 

 dune we had seen lying on the eastern horizon for one or two days before 

 reaching the chott. 



