FORMATION OP THE ALIOS, 85 



protected by old forests, have never been swept by the winds for 

 centuries. Soundings, and the knowledge of these exceptions, led M, 

 Faye to the discovery of the mode in which the alios was formed. 

 In winter and early spring the nearly level surface of the Landes is 

 covered with rain-water, but during summer the level of this water 

 descends by evaporation, to the depth of one or two mfetres, a level 

 which also corresponds with that of the ponds which border the chain 

 of downs. If now we take into consideration the decomposition of 

 vegetable matter which takes place in the water, and the deposit 

 which must be produced at the lower level, it is easy to see why an 

 agglomeration of sand and organic matter should take place at the 

 depth already mentioned. This operation being repeated annually 

 during many centuries, an increasing stratum of alios is naturally 

 formed, which doubtless continues to grow at the present moment. 



" It is not surprising then that no alios is to be found in the 

 marshes which are always under water, nor in the downs which are 

 not inundated, like the Landes, by a periodical sheet of water carried 

 off regularly by evaporation, the rain as it falls being carried away 

 by the slopes to the sea. 



" But Whence come the traces of ferruginous matter which aid in 

 the agglomeration of the alios and in giving it its red tint 1 It was 

 shown long ago that the decomposition of roots and other vegetable 

 matter brings the peroxide of iron contained in the soil into a state 

 of inferior oxidation, and renders it liable to be attacked by the weak 

 acids resulting from vegetable decay; more recently, M. Daubree 

 attributed the formation of the limonitic iron of the Swedish lakes to 

 this chemical action, showing that iron thus rendered soluble over 

 great areas is collected together by springs and rivulets, re-assumes 

 its primitive oxidation, when the waters come in contact with the air, 

 and is then deposited in the form of slime, and forms mineral strata 

 of great richness. The same effect, but produced on the spot, would 

 account for the small quantity of iron found in the alios. Vegetable 

 decay has, in fact, produced in places the identical efiFect on the 

 blackish portions of the sand of the Landes ; where a fall in the level 

 has caused a great accumulation of water there has been a concen- 

 tration of iron, and in past times a certain number of furnaces worked 

 up the iron, which is now exhausted. 



" M. Faye, having explained the origin of alios, showed what eflfect 

 an impermeable subsoil has on the salubrity of a district ; the escape 

 of the water is stopped, the subsoil becomes a centre of putrefaction 

 and infection, and endemic malaria devastates the country. In the 



