SAFDS OF EGYPT. 531 



But even in the cases wliere the aceumulations of sand in ex- 

 tensive deserts appeal- to be of mai-ine formation, or rather aggre* 

 gation, and to have been brought to their present position by 

 upheaval, they are not wholly composed of material collected or 

 distributed by the currents of the sea ; for, in all such regions, 

 they continue to receive some small contributions from the dis- 

 integration of the rocks which underlie, or crop out through, the 

 superficial deposits.* In some instances, too, as in Northern 

 Africa, additions are constantly made to the mass by the preva- 

 lence of sea-winds, which transport, or, to speak more precisely, 

 roll the finer beach-sand to considerable distances into the in- 

 terior. But this is a very slow process, and the exaggerations of 

 travellers have diffused a vast deal of popular error on the sub- 

 ject. 



Scmds of Egyjpt. 



In the narrow valley of the Nile — ^which, above its bifurcation 

 near Cairo, is, throughout Egypt and Nubia, generally bounded 



upon its northern shores is less than at some former periods, though no ex- 

 tensive series of observations on this subject has been recorded. On the Spit 

 of Agger, at the present outlet of the Liimf jord, Andresen found the quan- 

 tity during ten years, on a beach about five hundred and seventy feet broad, 

 equal to an annual deposit of an inch and a half over the whole surface. — Om 

 Klitformationen, p. 56. 



This gives seventy-one and a quarter cubic feet to the running foot — a 

 quantity certainly much smaller than that cast up by the same sea on the 

 shores of the Dano-German duchies and of Holland, and, as we have seen, 

 scarcely one-fourth of that deposited by the Atlantic on the coast of Gascony. 



* See, on this subject, an article in Aus der JVdiur, vol. xxx., p. 590. 



The Florentine Frescobaldi, who visited the Sinaitic peninsula five hundred 

 years ago, observed the powerful action of the solar heat in the disintegration 

 of the desert rocks. " This place," says he, " was a ridge of rocks burnt to 

 powder by the sun, and this powder is blown away from the rock by the wind 

 and is the sand of the desert ; and there be many hills which are pure bare 

 rock, and when the sun parcheth them, the wind carries oflf the dust, and 

 other sand is there none in that land." — Viaggio, pp. 69, TO. 



In Arabia Petraea, when a wind, powerful enough to scour down below the 

 ordinary surface of the desert and lay bare a fresh bed of stones, is followed 

 by a sudden burst of sunshine, the dark agate pebbles are often cracked and 

 broken by the heat ; and this is the true explanation of the occurrence of the 

 fragments in situations where the action of fire is not probable. If the frag- 

 ments are small enough to be rolled by the winds, they are in time ground 

 down to sand and contribute to the stock of that material which covers the 

 face of the desert, though the sand thxis formed is but an infinitesimal pro- 

 portion of the whole. 



