Two chotts were encountered between Ghardaia and Ouargla, one unimportant, 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 unbroken 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. The relation of the Chott Mellala to the country to the north or the south was not seen, but between it and Ouargla there lies a succession of smaller and more irregular chotts, which together form a fairly well-connected chain. These chotts are separated by low passes and flat-topped hills whose summits are on a level with the neighboring plain. Many of the hills are cone-shaped and in other topographic features the region shows the eroding action of wind. About 7 kilometers from Ouargla an opening in the mamelons gives a view of a plain extending on a lower level to the horizon. This is the reg, or fluvial desert. The Ouargla plain, or reg, is connected with the drainage of the great Oued Ighaghar and has a character which in many ways is different from the Gantara, over which we had just passed. With the descent to the reg desert a more monotonous region is encountered. In the vicinity of Ouargla and for some kilometers to the north the topography is quite flat and gives the impression of a flood-plain. To the east it stretches unbroken to the horizon, but to the west it is bounded by a fairly abrupt wall, the Gantara escarpment. At intervals of several kilometers low sand ridges cross the route over the plain, and on the second day somewhat higher ground was traversed and a sand ridge about 4 kilometers across was encountered. The country then becomes somewhat more broken and presents the appearance of being the remains of an ancient and more elevated plain. About 56 kilometers from Ouargla are the largest dunes crossed; where traversed, these were 10 kilometers from north to south and extended beyond our vision both to the east and the west. This is apparently the edge of extensive dune regions which lie mainly east of Touggourt and of the Oued Rirh. For possibly the last 30 kilometers of the journey to Touggourt there are dunes and chotts in alternation.