28 The Sahara. 
covering a crust of gypsum, which forms a true geological horizon. 
Some of these pebbles, small and well-rounded, are of chalcedony, 
others of limestone, and others of opaque silicates. They do not ex- 
tend beyond the Chott, and are consequently limited to the neighbour- 
hood of the mountain. This pebbly plain is not completely bare, for 
tufts of various plants grow here and there, which appear to accom- 
modate themselves perfectly to the soil and the climate. Amongst 
these are Ephedra fragilis, the connecting link between the Eguzse- 
taceé and the Conifere, and which appear to play, in the desert, 
the part of the Pinus mugho in our Alps. As a common plant, 
robust and sturdy, its creeping roots, finding little vegetable soil, 
search for it at a distance by extending to an extraordinary length 
from the stem. Besides Brooms, Pistachios, and Tamarisks, we fre- 
quently observed a large Grass, a species of Stypa, several feet in 
height, known among the Arabs by the name of ‘ Alfa.’ This is a 
useful plant, serving not only for food for horses and camels, but for 
the manufacture of besoms, mats, hats, bowls, and basins for holding 
milk, water, &c. For the traveller the Alfa is a wearisome vegeta- 
tion at a distance; and, as M. Fromentin remarks, it looks like an 
immense harvest which does not ripen, and withers without turning 
to gold colour ; while near, it is a maze of endless zig-zag meander- 
ings where the traveller stumbles at everystep. The soil is greyish, 
sandy, and averse to all other vegetation, except when refreshed by 
occasional rains ; for we found on our return, after some days of 
rain, that the plateau between Wady Rir and Biskra was covered 
with a quantity of young plants; vegetable life was awakened, and, 
though it was December, the country wore the aspect of spring. 
Desert of Erosion.—This phase of desert is characterized by enor- 
mous erosions, and a soil saturated with salt. I may cite as an 
evidence of these erosions, a river near Biskra, Wady Djeddi, the 
bed of which is many kilometres in width, though in ordinary cir- 
cumstances the water is almost entirely wanting. But when the 
water is high, for want of a defined channel, it diverges and spreads 
to right and left over an immense surface, and produces the most 
extraordinary erosions. This is owing to a bed of gypsum, which 
near the surface forms a sort of flooring, and, being hard, prevents 
the water from scooping a deep bed. ‘The ground has all the ap- 
pearance of excellent arable land, but in fact it is absolutely barren. 
Nothing will grow on it, owing to the salt which is mixed with the 
soil to an extraordinary degree; and this is connected with the 
neighbourhood of the Lakes (ehotts), which are themselves the re- 
mains of an ancient sea. ‘The salt-lands can at once be recognized 
by the fact that the horses’ hoofs raise no dust; and a numerous troop 
may trot over it as if it were the swept floor of a barn. This is 
especially striking after riding over a sandy district where the tra- 
veller has suffered from the dust: all at once it disappears, and he 
finds himself on the salt desert. ‘The quantity of salt is so great, 
and it absorbs so much water during the night, that the soil retains 
its humidity the greater part of the day. In those spots where the 
salt is not in such excess as to exclude all vegetation, there is a 
