ORIGIN AND FORMATION OF SOILS. 135 
is strong reason to assume the existence of compounds 
analogous to zeolites in every soil. 
e. Serpentine and Magnesian Silicates are generally 
slow of decomposition, and yield a meager soil. 
J. 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 flow 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. 
INCORPORATION 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 preadamite 
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. 
éool down to the ordinary temperature. For many years: 
