DECEMBER 27, 1906 | 
NATURE 
201 
its plateaux, its mountains, its profile, are all well 
brought before us, the illustrations are admirably 
reproduced, and the maps and plates make 
text clear. Naturally M. Foureau had ample 
opportunity for studying the effect of wind erosion. 
On the Sahara its work is patent. The great 
variation in temperature by day and night brings 
about a constant cracking and crumbling of 
rocky corners, grinding each other into smaller frag- 
ments, which are ever being blown about by winds 
powerful enough to whirl along lumps of stone like 
feathers. 
goes on, the friction of the particles of which gives 
to hard, compact rocks a polish like that of the 
lapidary’s wheel. Other rocks of unequal consistency 
yield irregularly. The author traces the effects on 
various kinds of rock, granites, and sandstone, and 
shows the fret and honeycombing that follow. 
Having produced the sand by hard wear and tear, it 
is comparatively easy to construct the dunes, so con- 
spicuous a feature on the comparatively level plains 
of the Sahara. Wherever obstructions intervene, 
such as prominent rocks, bushes, inequalities in the 
{ 
| evaporation ; 
Thus the constant manufacture of sand | 
distribution of sand-level, a steep talus of grains | 
gathers in the sheltered lee, while a more gentle 
sloping bank gradually rises on the windward side of 
the obstruction until this is finally buried. The weird 
Fic. 2.—Weathered pegmatite, showing the appearance of rock masses in the Sahara. 
shapes into which this sand is blown, the strange 
curves it assumes, seem to be in many parts of the 
Grand Erg the only variation in a desolate landscape 
(Fig. 1). The author’s description of Lake Chad is 
very interesting. This lake has about it something 
mysterious, and it is much to be regretted that the 
examination was not more thorough, with the view 
of unravelling what is obscure. To picture the lake 
as a compact sheet of water is quite inadequate. 
Whether the lake may be considered as containing 
an archipelago, or whether it runs away _ into 
numerous lagoons, creeks, swampy stretches, dis- 
connected from the main body, is not yet cleared up. 
Whether these detached patches of water were 
originally parts of the lake, and now represent the 
deepest portions of the original bed, indicating that 
the lake is gradually disappearing, is one of those 
problems upon which more information is required. 
In these detached lalelets the water is brackish, but 
in the main body of the lake the water is fresh. 
The author raises the point whether some of these 
separate lagoons are not fed by subterranean water, 
which might explain the presence of salt. Where 
communication with the lake is probable or suspected 
there is very little salt in the water, but where the 
two are clearly distinct the presence of salt is very 
NO. 1939, VOL. 75] 
pronounced in the smaller body. M. Foureau directs 
attention to some very interesting points concerning 
the | the currents in this lake or in extensions of it, and 
discusses whether these currents are due to wind or 
but into this, as into many other 
important points examined in these volumes, we have 
not space to enter. Nie JOR Ie 
(2) Now that so much new light has been thrown 
on North African geology by the explorations in the 
Lake region in the east, on the one hand, and in 
the territories of Germany, France, Great Britain, 
and the Congo Free State in the west, the largest 
of the unexplored tracts left of the once ‘‘ Dark 
Continent’ is that of the French Sahara, lying 
between Algeria on the north and the Congo terri- 
tories on the south. Of this vast area a preliminary 
survey has been accomplished by the Mission 
Foureau-Lamy, which, setting out from Algeria in 
November, 1898, reached the Congo by way of Lake 
Chad in July, 1900. Ample collections were made 
along this almost unknown route, and detailed 
observations recorded on the topography, hydrology, 
and geology of the countries on the line of march. 
The results of the examination of the botanical, zoo- 
logical, geological, and ethnological specimens, in the 
fight of observations made during the journey, are 
now published by the Geographical 
Society of France with the aid of 
subventions made by the French 
Government, the Academy, and the 
French Association for the Advance- 
ment of Science. Needless to say, 
the work before us (part iii.) has 
been issued in a form worthy of the 
highest traditions of French science, 
and with a great wealth of valuable 
illustrations. 
In the geological notes made by 
the travellers along the line of route 
are found many very interesting 
observations on the mode of weather- 
ing of rocks in tropical districts. 
The illustrations reproduced are ex- 
cellent examples of the action of sun 
and wind upon rocks in a desert 
region. The rocks passed over con- 
sist of granite, which appears to 
occupy a very large area, crystalline schists, and 
sporadic masses of various volcanic rocks, with re- 
presentatives of Silurian, Devonian, Carboniferous, 
and Cretaceous formations. In addition to these, 
there are sandstones of which the geological age 
could not be determined, and various superficial 
deposits. 
The rock specimens were entrusted to M. L. Gentil, 
of the Sorbonne, a pupil of Prof. Lacroix, for de- 
scription, and his report on the petrography of the 
regions traversed is a contribution of great value. 
Interesting laterites, diatomaceous earths, and traver- 
tines are among the most important of the materials 
of aqueous origin. The igneous rocks exhibit a 
great variety, and include, besides many varieties of 
granite, ophitic diabases, andesites, rhyolites, tra- 
chytes, phonolites, tephrites, and basalts. One very 
interesting feature exhibited by many of these rocks 
is their richness in the alkalies—the soda augites and 
hornblendes, like riebeckite, agerine, &c., abounding 
in them. These facts, taken in conjunction with the 
studies by Mr. Prior, of the British Museum, on the 
rocks of the Lake district of Africa, of Prof. Bonney 
on those of Socotra, and of Prof. Lacroix in the 
Somali country and Madagascar, lend support to the 
view enunciated by the last-mentioned geologist that 
