is, coelumque profundum.

BOOK II.

CHANGES IN THE INORGANIC WORLD.

AQUEOUS CAUSES.

CHAPTER XIV.

Division of the subject into changes of the organic and inorganic
world--Inorganic causes of change divided into aqueous and
igneous--Aqueous causes first considered--Fall of rain--Recent
rain-prints in mud--Destroying and transporting power of running
water--Newly formed valleys in Georgia--Sinuosities of rivers--Two
streams when united do not occupy a bed of double
surface--Inundations in Scotland--Floods caused by landslips in the
White Mountains--Bursting of a lake in Switzerland--Devastations
caused by the Anio at Tivoli--Excavations in the lavas of Etna by
Sicilian rivers--Gorge of the Simeto--Gradual recession of the
cataract of Niagara.

_Division of the subject._--Geology was defined to be the science which
investigates the former changes that have taken place in the organic as
well as in the inorganic kingdoms of nature. As vicissitudes in the
inorganic world are most apparent, and as on them all fluctuations in
the animate creation must in a great measure depend, they may claim our
first consideration. The great agents of change in the inorganic world
may be divided into two principal classes, the aqueous and the igneous.
To the aqueous belong Rain, Rivers, Torrents, Springs, Currents, and
Tides; to the igneous, Volcanoes, and Earthquakes. Both these classes
are instruments of decay as well as of reproduction; but they may also
be regarded as antagonist forces. For the aqueous agents are incessantly
laboring to reduce the inequalities of the earth's surface to a level;
while the igneous are equally active in restoring the unevenness of the
external crust, partly by heaping up new matter in certain localities,
and partly by depressing one portion, and forcing out another, of the
earth's envelope.

It is difficult, in a scientific arrangement, to give an accurate view
of the combined effects of so many forces in simultaneous operation;
because, when we consider them separately, we cannot easily estimate
either the extent of their efficacy, or the kind of results which they
produce. We are in danger, therefore, when we attempt to examine the
influence exerted singly by each, of overlooking the modifications which
they produce on one another; and these are so complicated, that
sometimes the igneous and aqueous forces co-operate to produce a joint