s, and Tierra del
Fuego, we meet with an abundant growth of this substance. Almost all
plants contribute here by their decay to the production of peat, even
the grasses; but it is a singular fact, says Mr. Darwin, as contrasted
with what occurs in Europe, that no kind of moss enters into the
composition of the South American peat, which is formed by many plants,
but chiefly by that called by Brown _Astelia pumila_.[1007]

I learn from Dr. Forchhammer (1849) that water charged with vegetable
matter in solution does not throw down a deposit of peat in countries
where the mean temperature of the year is above 43 degrees or 44 degrees
Fahrenheit. Frost causes the precipitation of such peaty matter, but in
warm climates the attraction of the carbon for the oxygen of the air
mechanically mixed with the water increases with the increasing
temperature, and the dissolved vegetable matter or humic acid (which is
organic matter in a progressive state of decomposition) being converted
into carbonic acid, rises and is absorbed into the atmosphere, and thus
disappears.

_Extent of surface covered by peat._--There is a vast extent of surface
in Europe covered with peat, which, in Ireland, is said to extend over a
tenth of the whole island. One of the mosses on the Shannon is described
as being fifty miles long, by two or three broad; and the great marsh of
Montoire, near the mouth of the Loire, is mentioned, by Blavier, as
being more than fifty leagues in circumference. It is a curious and
well-ascertained fact, that many of these mosses of the north of Europe
occupy the place of forests of pine and oak, which have, many of them,
disappeared within the historical era. Such changes are brought about by
the fall of trees and the stagnation of water, caused by their trunks
and branches obstructing the free drainage of the atmospheric waters,
and giving rise to a marsh. In a warm climate, such decayed timber would
immediately be removed by insects, or by putrefaction; but, in the cold
temperature now prevailing in our latitudes, many examples are recorded
of marshes originating in this source. Thus, in Mar forest, in
Aberdeenshire, large trunks of Scotch fir, which had fallen from age and
decay, were soon immured in peat, formed partly out of their perishing
leaves and branches, and in part from the growth of other plants. We
also learn, that the overthrow of a forest by a storm, about the middle
of the seventeenth century, gave rise to