OKIGIN AXD FOUMaTION OF SOILS. 13S 



is strong reason to assume tlie existence of compounds 



analogous to zeolites in every soil. 



e. Serpentine and Maja^ncsian Silicates are generally 

 slow of decomposition, and yield a meager soil, 



/. The Limestones, when pure and compact, are very 

 durable : as they become broken, or when impure, they 

 often yield rapidly to the weather, and impregnate the 

 streams which How over them with carbonate of lime. 



g. Argillite and Argillaceous Limestones, which have 

 resulted from the solidification of clays, readily yield clay 

 again, either by simple pulverization or by pulverization 

 and weathering, according as they have suffered more or 

 less change by metamorphism. 



§5. 



JNCORPORATION OF ORGANIC MATTER WITH THE SOIL AND 



ITS EFFECTS. 



Antiquity of Vegetation. — Geological observations lead 

 to the conclusion that but small portions of the earth's 

 surface-rocks were formed previous to the existence of 

 vegetation. The enormous tracts of coal found in every 

 quarter of the globe are but the residues of preadaraite 

 forests, while in the oldest stratified rocks the remains of 

 plants (marine) are either most distinctly traced, or the 

 abundance, of animal forms warrants us in assuming the 

 existence of vegetation previous to their deposition. 



The Development of Vegetation on a purely Mineral 

 Soil. — The mode in which the original inorganic soil be- 

 came more or less impregnated with organic matter may 

 be illustrated by what has happened in recent years upon 

 the streams of lava that have issued from volcanoes. The 

 lava flows from the crater as red-hot molten rock, often in 

 masses of such depth and extent as to require months to 

 cool down to the ordinary temperature. For many years 



