84 SAND-WASTES OF FBANOB. 



but great care has to be taken not to fracture it, as whenever this 

 happens the water is lost, draining off to a lower depth. 



In the Landes the stratum of impermeable matter spoken of by 

 Herr Wessley, and there known as alios, contains iron in its composi- 

 tion ; it has been spoken of as bog-iron-ore, and has been mentioned 

 (ante p. 66) under this name among the products of the Landes 

 exhibited by M. Leopold Javal. 



The origin of this has been discussed by M. Faye, Director-General 

 of the Administration of Forests in France, in a paper which appeared 

 in the translation of the Academie des sciences, from a notice of 

 which, in the Aihenmwm, it appears that M. Faye was engaged to 

 level a portion of the Landes between the lakes on the coast 

 and the basin of Arcachon, and made use of the opportunity to 

 study the peculiarities of the soil. According to this notice : " At 

 about three feet below the surface of the Landes, there lies every- 

 where an impermeable stratum called alios, a stony substance of a 

 brown colour, variable in thickness, which is nowhere great, and 

 covering an indefinite bed of sand, identical with that which lies above 

 it. This invisible waterproof stratum has always had a great influence 

 on the health of the inhabitants of the country. Retaining the pro- 

 ducts of vegetable decomposition from the upper soil, where there 

 was scarcely any slope, the alios has for centuries fixed intermittent 

 fever in and around the Landes ; but reclamation has driven away 

 the fever, and the alios seems now to have no other effect than that 

 of forcing the roots of the marine pines to grow horizontally instead 

 of vertically. The sand of the Landes is white, intermixed with a 

 few black grains, containing peroxide of iron and oxide of manganesia. 

 Washed, first by the water of the ocean, and afterwards by rain for 

 centuries, it holds no soluble matter, and the alios, which is of a dark 

 reddish brown colour, sufficiently compact to require a pick-axe to 

 break it up, is a stratum of the same sand cemented together by some 

 organic and slightly ferruginous substance. In the summer a hole 

 made in the soil down to the alios fills gradually by lateral infiltration 

 with yellowish water not fit for drinking ; but if the alios is pierced 

 an abundant supply of perfectly limpid water is obtained. 



" The question is — How is this alios formed 1 It is evident that it 

 was produced in situ, and the presence of the organic matter already 

 mentioned leads to the supposition that the latter plays some part in 

 the formation ef this peculiar stratum. 



" The alios is found everywhere in the Landes except in the marshes, 

 on the banks of ponds, and in the downs, even when the latter, 



