1270 General Notes. [December, 
continues the Erg group to the south-west into Morocco, and 
that of Edeyen to the south-east of Erg. The Erg group ex- 
tends from the 20° to 34° N. lat, and from 7° E. long., to 4° W. 
long. Erg alone is reckoned to occupy 12,000,000 hectares, 
or about 45,000 square miles, but the estimate is probably too 
large, as immense spaces within the area are free from dunes. 
The dunes are in some places piled into chains of sand mountains, 
which may reach several kilometers in width, and 500 to 60 
feet in height. The true dune, when not piled on other dunes, is 
of uniform composition and regular form. The grains are usually 
less than a millimeter in diameter, and the shape of the dune is 
an elongated ellipse, with a concavity cut out of the leeward 
side. The sand, driven by the wind, climbs up the long gradual 
slope of the ellipse, and falls over the abrupt talus of the short 
concave side, which is bounded above by a sharp edge. A sim- 
ple dune seldom exceeds sixty-five feet in height, but here and 
there one rises to more than two hundred feet. 
The dunes occupy basins of Quaternary age, and have. been 
formed by the disintegration of rocks of various ages. Disinte- 
gration proceeds less rapidly in a dry climate than in a wet one, — 
but in the Sahara there is no vegetation to protect the a : 
and the disintegrated material is never consolidated into . 
The chief causes of rock disintegration in the Sahara y, m ‘ 
great difference of temperature, amounting often t -i 
tween the day and the night, and the action of wind-blown ee 
upon the rocks; chemical action and the infrequent rains may TG 
d 
added. l r EE 
Comali-land—The Geographical Society of Paris ef fy 
published the results of the journey to the country © wpe 
malis, undertaken by M., Revoil in 1880. The region may p 
divided into three zones, the coast, where the towns are al in 
the mountains, which are often calcareous and are ogee | 
their stratification with those along the borders of yi docks. 
and the interior plateau, inhabited by nomads with fer there by 
k 
he 
bed of blackish siliceous sand. These steppes ar patie 
immense pastures, affording subsistence to the onnan the 
oxen, sheep, goats, asses, horses and camels whic pe <a 
only riches of the Comalis of the interior. Most se par S 
are torrents of short course, and the only river | 
pastures here and there. The climate is tempera 
34° C. on the coast, and to 45° or even 55 
