EFFECTS OF FOREST YEaETATIOIf OIT CLIMATE. 203 



we find the following remarks, showing the effect of forest 

 destruction in creating a desert : — 



" Vice-Consul Dupuis, in his report this year on the trade of 

 the port of Susa, Tunis, makes remarks on the subject of the 

 project for submerging the region of Djerid by constructing a 

 canal at Gabes, and so creating an inland sea. He considers 

 that the recent surveys confute the idea of their having been 

 formerly a connexion with the Mediterranean, and of the choking 

 up of the passage for the waters, an idea perhaps based upon 

 the inferiority of level to that of the sea ; but in his opinion the 

 observations made seem to endorse the fact of all the region 

 having been under water. Arab writers unite in describing the 

 country at the date of their conquest as having been ver}^ wooded 

 and abundantly supplied with streams of water. The wood was 

 cut down to facilitate the subjection of the tribes, who for above 

 a century fought desperately for their independence, and whole 

 regions are now condemned to sterility (save, perhaps, an oasis 

 here and there), which were formerly rich in pastures, and inter- 

 spersed with towns. The desert has been gradually extended in 

 the district between Tripoli and Egypt, covering parts once 

 fertile, and has in like manner encroached on the Tunisian 

 southern frontier between it and Tripoli. The diminished heights 

 and lowering of the Atlas let in the sands driven by the southerly 

 winds, to which the more elevated and uniform heights of the 

 mountain system oppose a barrier in more favoured Barbary 

 States westward. In Morocco these winds are so tempered in 

 their passage across the intervening heights as hardly to be 

 recognized as the same which in Tunis dry up and parch the 

 land in summer. Their action upon the sands accumulated by 

 them at the foot of and in the passes of the mountains south- 

 ward, where they sink into the plain, is the same as that seen at 

 street corners, but on a large scale, and the sands are whirled 

 and spread over the southern provinces. A form of this in- 

 draught and encroachment is seen in the winds v\^hich predomi- 

 nate in the fall of the year and fill the air with a minute and 

 impalpable sand, very injurious to the sight ; this sand, on exami- 

 nation of collections of it, is found to be very fine, while that 

 around and in the valleys generally is coarse, the one being- 

 foreign or sands drifted from lon^ distances, and the other indi- 

 genous or formed on the spot. It is presumed that the disap- 

 pearance of the waters is due to the encroachment of the desert 

 caused by the action of these winds during a long succession of 

 centuries, aided by absorption and by evaporation occasioned by 

 the presence of the vast scorching desert on the south, and also 

 by the substances brought down by streams diminishing the depths 

 and spreading the waters, and thereby helping in the Avork of 

 desiccation. This was accelerated also by a decrease in the 



