13(5 HOW CR0Pt5 FEED. 



the lava is incapable of bearing atiy vegetation save some 

 almost microscopic forms. During these years the surface 

 of the rock suttVrs gradual disintegration by tlie agencies 

 of air and water, and so in time acquires the power to 

 support some lichens that appear at first as mere stains 

 upon its surface. These, by their decay, increase the 

 film of soil from which they sprung. The growth of 

 new generations of these p-lants is more and more vigor- 

 ous, and other superior kinds take root among them. 

 After another period of years, there has accumulated a 

 tangible soil, supporting herbaceous plants and dwarf 

 shrubs. Henceforward the increase proceeds more rapid- 

 ly ; shrubs gradually give place to trees, and in a century, 

 more or less, the once hard, barren rock has M'eathered to 

 a soil fit for vineyards and gardens. 



Those lowest orders of plants, the lichens and mosses, 

 which pi-epare the way for forests and for agricultural 

 vegetation, are able to extract nourishment from the most 

 various and the most insoluble rocks. They occur abund- 

 antly on all our granitic and schistose rocks. Even on 

 quartz they do not refuse to grow. The white quartz 

 hills of Berkshire, Massachusetts, are covered on their 

 moister northern slopes with large patches of a leathery 

 lichen, which adheres so firmly to the rock that, on being 

 forced off", ])articles of the stone itself are detached. Many 

 of the old marbles of Greece are incrusted with oxalate 

 of lime left by the decay of lichens Avhich have grown 

 upon their surface. 



Humus. — By the decay of successive generations of 

 plants the soil gradually acquires a certain content of dead 

 organic matter. The falling leaves, seeds and stems of 

 vegetation do not in general waste from the surface aa 

 rapidly as they fwe renewed. In forests, pastures, prai- 

 ries, and marshes, there accumulates on the surface a brown 

 or black mass, termed humus, of which leaf mold, swamp- 

 muck, and peat are varieties, differing in appearance as in 



