THE SURFACE OF THE GLOBE. 87 



lands in their vicinity, whose progress towards the sea they intercept, 

 and their advance in many places is made with alarming rapidity. 

 Forests, buildings, and cultivated fields, are overwhelmed by them. 

 Those of the Bay of Biscay* have already covered a number of 

 villages mentioned in the accounts of the middle ages, and, at this 

 time in the single department of Landes, they are threatening to 

 advance with inevitable destruction. One of these villages, that of 

 Mimisan, has struggled against them for twenty years, and a down 

 more than sixty feet high is perceptibly approaching it. 



In 1S02 the pools overflowed five fine farms in the village of St. 

 Julien.f They have long since covered an ancient Roman road 

 leading from Bourdeaux to Bayonne, and which could be seen forty 

 years ago when the waters were low+. The Adour, which was 

 known to have formerly passed Old Boucaut, and flowed into the sea 

 at Cape Breton, is now turned from it more than a thousand fathoms. 



The late M. Bremontier, inspector of bridges and roads, who made 

 great researches on downs, calculated their progress at sixty feet an- 

 nualy, and in some places at seventy-two. According to his calcula- 

 tions, they will reach Bourdeaux in two thousand years ; and from their 

 present size rather more than four thousand years must have elapsed 

 since their accumulation commenced§. The overwhelming of the 

 cultivated lands of Egypt by the sterile sands of Libya, which the 

 west wind casts on them, is a phenomenon similar to that of the downs. 

 These sands have buried a number of cities and villages, whose ruins 

 may still be seen, and that since the conquest of the country by the 

 Mahometans, as the tops of mosques and the pinnacles of minarets 

 are to be seen projecting through the sand,|| Advancing so rapidly 

 they would doubtlessly have filled the narrow defiles of the valley if 

 so many ages had elapsed since they began to be cast there. ^f There 

 would be nothing left between the Libyan chain and the Nile, It is 

 then a chronometer, the measure of which it would be as easy as inte- 

 resting to obtain. 



Turf-bogs and Slips. 



The turf-bogs, so generally produced in the north of Europe by the 

 accumulation of the remains of sphagna and other aquatic mosses, also 

 give us a measure of time. They increase in proportion determined 



* See the Report of the Downs of the Bay of Biscay by M. Tassin, Mont de Mar- 

 san, an. X. 



t Memoirs of M. Bremontier, of the fixing of Downs, X Tassin loc. Cit. 



§ See Bremontier's Memoir. || Denon — Voyage en Egypte. 



^f We may here refer to all travellers who have traversed the western parts of 

 Egypt. 



