536 THE POPULAR SCIENCE MONTHLY. 



material added by the waves ; then the high dunes back of them, 

 which, receiving the drift-sand, gradually rise in height and be- 

 come better fitted to protect the mainland against wind and sand ; 

 third, the inner dunes, a series of low hills lying back of the high 

 dunes, which are formed of the sand that is blown over the high 

 dunes. [This division is, however, theoretical, and not always 

 recognizable in the fact.] 



The two sides of the dune slope at different angles. On the 

 windward side the rise is gradual, at an angle seldom of more 

 than from 5° to 10°. But the other side is much steeper, and usu- 

 ally offers an angle of 30°. The variations that appear are ex- 

 plained by differences in the cohesion of the sand ; coarse sand 

 forms a steeper slope than fine sand. The dunes have an average 

 height of some forty-five or fifty feet ; but there are some of great- 

 er altitude. In Jutland, for example, and on the Frisian and 

 Courland low ground, and in the Landes, they rise to from one 

 hundred and fifty to two hundred feet. In breadth, they vary 

 from three hundred to three thousand feet ; but they reach an ex- 

 traordinary development in the Landes, where breadths of thirty 

 thousand feet are not rare. 



The large sand stretches in the interior were formerly sea-bot- 

 toms. Another origin has been sought for the Banat sands of 

 the Hungarian steppe, in the supposition that they are a product 

 of the Danube, which, tremendously disturbed by southeastern 

 storms in the vicinity of Palanka, has thrown its sand-weighted 

 spray into the air to be carried far inward, dropping its heavier 

 constituents along the way. This hypothesis needs to be men- 

 tioned only to be contradicted. 



It can hardly be supposed that an inland drift-sand district 

 has lain bare during historical time, or since that long-past epoch 

 when the retreating waters left it dry. On the contrary, sand 

 constantly acquires an increasing verdancy through the unceasing 

 efforts of Nature, and becomes at last covered with a thick carpet 

 of the lower plants, or with wood, and thereby fully protected 

 against the wind. This gradual process of binding goes on the 

 more speedily when it is not interrupted by man. The presence 

 of a former coating of humus is easily proved by digging into the 

 sand ; for, everywhere that the sand-hills are removed, dark- 

 colored strata appear, which were certainly once surface -soil 

 covered with plants. The wind-blowings occasionally bring to 

 light carbonized relics and reed-shaped holes which are unmistak- 

 ably derived from tree-roots. Cylindrical massive or hollow tufa 

 formations are also observed, the shapes, direction, and ramifica- 

 tions of which likewise point to roots ; and they have probably 

 been formed from roots through the infiltration of calciferous 

 water. 



