96 SAND-WASTES OP FRANCE. 



amount to J° or 1° Fahr., and at that depth, at the elevation of about 

 300 feet above the sea level, the temperature is almost stationary at 

 10° R., or 54£° Fahr. 



By directing our attention to the sand drifts of Sologne, and the 

 sand drifts of the Landes of Gasoony, we find we have had two 

 different phases of sand drifts brought under our consideration ; and 

 as these two phases of these are to be met with again and again, 

 sometimes in the same countries elsewhere, or the one and the other 

 of them presenting themselves in lands which are far apart, the 

 opportunity may be taken, before proceeding further, to consider 

 at some length an important point in which they differ. 



Of the two forms of sand deposit thus brought under considera- 

 tion, " The one," to quote Marsh, " is that of dune or shifting 

 hillock upon the coast; the other that of barren plain in the 

 interior. The coast dunes are composed of sand washed up 

 from the depths of the sea by the waves, and heaped in more 

 or less rounded knolls and undulating ridges by the winds. The 

 sand with which many plains are covered appears sometimes to 

 have been deposited upon them while they were yet submerged 

 beneath the sea ; sometimes to have been drifted from the 

 sea coast and scattered over them by wind currents ; sometimes to 

 have been washed upon 'them by running water. In these latter 

 cases, the deposit, though in itself considerable, is comparatively 

 narrow in extent, and irregular in distribution, while in the former 

 it is often evenly spread over a very wide surface. 



"In all great bodies of either sort, the silicious grains are the 

 principal constituent, though, when not resulting from the disintegra- 

 tion of silicious rocks and still remaining in place, they are generally 

 accompanied with a greater or less admixture of other mineral particles, 

 and of animal and vegetable remains; and they are also usually 

 somewhat changed in consistence by the ever varying conditions of 

 temperature and moisture to which they have been exposed since their 

 deposit. Unless the proportion of these latter ingredients is so large 

 as to create a considerable adhesiveness in the mass, in which case 

 it can no longer properly be called sand, it is infertile, and, if not 

 charged with water, it is partially agglutinated by iron, lime, or other 

 cement, or confined by alluvion resting upon it. It is much inclined 

 to drift, whenever by any chance the vegetable network which in 

 most cases thinly clothes it, and at the same time confines it, is broken. 

 " Human industry has not only fixed the flying dunes by plan- 

 tations ; but by mixing clay and other tenacious earths with the 



