6 MODERN tfO&ESt ECONOMY. 



extensively accepted by scientific men, and the truth of 

 which, so far as is known to me, has never been called in 

 question. In an address delivered before La Ligue du 

 lieboisement de U Algerie, by Dr Tholard, the founder of 

 the league, there was given the following graphic state- 

 ment of the views in question : 



'When the mountains arose in the geological revolutions 

 which gave to them birth, they must have presented them- 

 selves in the condition of bare rocks, or banks of solidified 

 matter, without trace of vegetation. But soon thereafter 

 moisture in the atmosphere, deposited on their surface, 

 penetrated into fissures, and there expanding in passing 

 from the liquid to a solid state with a fall of temperature, 

 burst asunder the walls of the interstices into which they 

 had infiltrated, and the work of the disintegration of the 

 rock began. This physical action of the water was followed 

 by a chemical action due to carbonic acid, with which it 

 is always charged, which, combining with the alkaline 

 elements of the rocks, decomposed and dissolved them, 

 leaving a skeleton of rock more easily penetrable by 

 water; and, as a consequence, facilitating its further dis- 

 integration. Thus there was produced, self-formed, a layer 

 of vegetable soil, or soil capable of sustaining vegetation, 

 resting almost immediately on the rock as yet unaffected. 

 Then in many places became developed the first formed 

 vegetables, the germs of which had been borne thither by 

 the air ; but these, doubtless, were vegetables of simple 

 organisation, the conditions not being as yet favourable to 

 the germination and growth of plants of more complex 

 structure. However low in the scale of vegetable organi- 

 sation these might be, they continued or carried further 

 the work of disintegration begun by the atmospheric 

 moisture; their rootlets penetrated into the fissures of the 

 rock, and on the death of the plant these became them- 

 selves decomposed, leaving thus a path open for water, 

 which by degrees, like an indefatigable miner, continued 

 its work of crumbling down the rock. 



' By the decomposition of the rock thus effected, the 



