MAECH OF THE DUNES, 203 



The high dunes of Lagrave, to the south of Archachon, are the most 

 curious in this respect ; below, the sea forces them to fall in ; above, 

 they bury the pine-trees in their invading masses of sand. 



The most favourable days for observing the progressive march of 

 dunes are those when a gentle breeze, strong enough however to 

 drive the sand before it, blows in a perfectly uniform manner. From 

 the top of the dune we see innumerable grains of dust swiftly scaling 

 the slope. Glittering in the sun, and whirling like the midges in a 

 fine summer's evening, they attain the summit, then accumulate in 

 the form of a cornice on the other side of the ridge, and from time to 

 time occur little falls, which spread over the surface of the talus, 

 like sheets of water over the sides of a rock, and whose contours re- 

 mind one of light draperies covering one another. When a high 

 wind blows with violence, and in successive gusts, the encroachments 

 of the dune are accomplished in a manner much more rapid, but 

 often much more difficult to observe. The summits of the hillocks, 

 which are enveloped in clouds of dust, resemble volcanoes vomiting 

 smoke ; the front of the dune is furrowed and scooped out by the 

 wind ; masses of sand, laden with marine remains brought by the 

 storm, fall down with an audible sound, and are disposed in unequal 

 layers over the descending talus. A section taken across a dune 

 would permit us to count and measure the different strata varying 

 in thickness and composition, which the winds have successively 

 brought. Here we find a fine sandlike dust ; there, a stronger 

 wind was charged with a heavy shelly sand ; while, again, a storm 

 has carried away entire shells, branches, and waifs. However, the 

 particles transported by the wind are, in general, all the finer the 

 further they are from the sea, and this is reasonable, for they must 



Fig. 91.— Section of a dune. 



fly more easily the less resistance they offer to the aerial current which 

 bears them. In the narrow rows of dunes which border certain parts 

 of the coast of the Mediterranean, we can clearly see over a breadth 

 of some hundreds of yards, the moving materials succeed each other, 

 distributed according to their weight. First, there are the frag- 



