STEATIFICATION OF DUNES. 549 



San Francisco, as well as in all coast-dunes and other accumida- 

 tions of loose mineral material in similar situations. Kohl ob- 

 serves that the shore on the landward side of the files of dunes 

 often trembles from the shock of the waves on the beach,* and 

 Yilleneuve estabhshed by careful experiment that at Dunkerque 

 the ground is sensibly agitated by the same cause, in stormy 

 weather, to a distance of more than a mile from the sea. 



The eddies of strong winds between the hillocks must also 

 occasion disturbances and rearrangements of the sand layers, and 

 it seems possible that the irregular thickness and the strange contor- 

 tions of the strata of the sandstone at Petra may be due to some 

 such cause. A curious observation of Professor Forchhammer sug- 

 gests an explanation of another peculiarity in the structure of 

 file sandstone of Mount Seir. He describes dunes in Jutland, 

 composed of yellow quartzose sand intermixed with black titanian 

 iron. When the wind blows over the surface of the dunes, it 

 furrows the sand with alternate ridges and depressions, ripples, 

 in short, like those of water. The swells, the dividing ridges of 

 the system of sand ripples, are composed of the light grains of 

 quartz, while the heavier iron rolls into the depressions between, 

 and thus the whole surface of the dune appears as if covered with 

 a fine black network. 



The sea side of dunes, being more exposed to the caprices of 

 the wind, is more irregular in form than the lee or land side, 

 where the arrangement of the particles is affected by fewer dis- 

 turbing and conflicting influences. Hence, the stratification of 

 the windward slope is somewhat confused, while the sand on the 

 lee side is found to be disposed in more regular beds, inclining 

 landwards, and with the largest particles lowest, where their 

 greater weight would naturally carry them. The lee side of the 

 dunes, being thus formed of sand deposited according to the laws 

 of gravity, is very uniform in its slope, which, according to 

 Forchhammer, varies little from an angle of 30° with the hori- 

 zon, while the more exposed and irregular weather side lies at an 

 inclination of from 5° to 10°. When, however, the outer tier of 

 dunes is formed so near the water line as to be exposed to the 

 immediate action of the waves, it is undermined, and the face of 

 the hill is very steep and sometimes nearly perpendicular. 



* Inseln und Marschen, etc., ii., p. 34. 



