480 
GEOLOGICAL IMPORTANCE OF DUNES. 
tory evidence as to their origin, their starting point, and the 
course by which they have wandered so far from the sea.* 
If the sand of coast dunes is, as Staring describes it, com¬ 
posed chiefly of well-rounded quartzose grains, fragments of 
shells, and other constant ingredients, it would often be recog¬ 
nizable as coast sand, in its agglutinate state of sandstone. 
The texture of this rock varies from an almost imperceptible 
fineness of grain to great coarseness, and affords good facilities 
for microscopic observation of its structure. There are sand¬ 
stones, such, for example, as are used for grindstones, where 
the grit, as it is called, is of exceeding sharpness ; others where 
the angles of the grains are so obtuse that they scarcely act at 
all on hard metals. The former may be composed of grains 
of rock, disintegrated indeed, and recemented together, but 
not, in the meanwhile, much rolled ; the latter, of sands long 
washed by the sea, and drifted by land winds. There is, 
* Forchhammer, after pointing out the coincidence between the in¬ 
clined stratification of dunes and the structure of ancient tilted rocks, 
says : “ But I am not able to point out a sandstone formation correspond¬ 
ing to the dunes. Probably most ancient dunes have been destroyed by 
submersion before the loose sand became cemented to solid stone, but we 
may suppose that circumstances have existed somewhere which have pre¬ 
served the characteristics of this formation.”— Leonhard und Bronn, 
1841, p. 8, 9. 
At the moment of sending my manuscript to the press, I find from 
Laurent {Memoir e sur le Sahara, etc., p. 12) that in the Algerian desert 
there exist “sandstone formations” not only “corresponding to the dunes,” 
but actually consolidated within them. “ A place called El-Mouia-Tadjer 
presents a repetition of what we saw at El-Baya; one of the funnels 
formed in the middle of the dunes contains wells from two metres to two 
and a half in depth, dug in a sand which pressure, and probably the pres¬ 
ence of certain salts, have cemented so as to form true sandstone, soft 
indeed, but which does not yield except to the pickaxe. These sand¬ 
stones exhibit an inclination which seems to be the effect of wind; for 
they conform to the direction of the sands which roll down a scarp occa¬ 
sioned by the primitive obstacle.” 
The dunes near the mouth of the Nile, the lower sands of which have 
been cemented together by the infiltration of Nile water, would probably 
show a similar stratification in the sandstone which now forms their base. 
