SOILS. 
BY D. O’BRINE, 
Chemist. 
Soils are formed by the natural disintegration of rocks, 
to which is added the black mould caused by the decay of 
animal and vegetable matter. This disintegration is 
assisted by the atmosphere. The oxygen of the atmos¬ 
phere is capable of uniting with some of the constituents 
of rocks, by which their cohesion is weakened or destroyed. 
This is the cause of the rapid disintegration of some varie¬ 
ties of granite. The iron is oxidized, its volume is in¬ 
creased, and portions of the rock are separated from the 
mass. When granite or limestone contains sulphuret of 
iron, the oxygen of the atmosphere, in the presence of 
moisture, combines with the sulphur and forms sulphuric 
acid, that decomposes limestone and the feldspar of 
granite. The carbonic acid of the atmosphere is another 
decomposing agent. Water charged with this gas is capa¬ 
ble of decomposing calcareous rocks; it is, for this reason, 
that caves occur in limestone formations. The moisture 
of the atmosphere has a decomposing action. Rocks 
which are exposed to frequent alternations of dryness and 
moisture soon crumble into fragments; in this connection, 
the mechanical action of the falling rain must not be for¬ 
gotten. Variations of temperature, especially above and 
below the freezing point, have great influence in the 
destruction of rocks. When a rock is saturated with 
water and the water freezes, it expands, and this expan¬ 
sion tends to enlarge the interstices and in time to sepa¬ 
rate particles of the rock. It is an observed fact that in 
