ith the fact, that minute particles
descend with extreme slowness through water, the extent of their surface
being very great in proportion to their weight, and the resistance of
the fluid depending on the amount of surface. A precipitate of sulphate
of baryta, for example, will sometimes require more than five or six
hours to subside one inch;[467] while oxalate and phosphate of lime
require nearly an hour to subside about an inch and a half and two
inches respectively,[468] so exceedingly small are the particles of
which these substances consist.

When we recollect that the depth of the ocean is supposed frequently to
exceed three miles, and that currents run through different parts of
that ocean at the rate of four miles an hour, and when at the same time
we consider that some fine mud carried away from the mouths of rivers
and from sea-beaches, where there is a heavy surf, as well as the
impalpable powder showered down by volcanoes, may subside at the rate of
only an inch per hour, we shall be prepared to find examples of the
transportation of sediment over areas of indefinite extent.

It is not uncommon for the emery powder used in polishing glass to take
more than an hour to sink one foot. Suppose mud composed of coarser
particles to fall at the rate of two feet per hour, and these to be
discharged into that part of the Gulf Stream which preserves a mean
velocity of three miles an hour for a distance of two thousand miles; in
twenty-eight days these particles will be carried 2016 miles, and will
have fallen only to a depth of 224 fathoms.

In this example, however, it is assumed that the current retains its
superficial velocity at the depth of 224 fathoms, for which we have as
yet no data, although we have seen that the motion of a current may
continue at the depth of 100 fathoms. (See above, p. 28.) Experiments
should be made to ascertain the rate of currents at considerable
distances from the surface, and the time taken by the finest sediment to
settle in sea-water of a given depth, and then the geologist may
determine the area over which homogeneous mixtures may be simultaneously
distributed in certain seas.

CHAPTER XXII.

IGNEOUS CAUSES.

Changes of the inorganic world, _continued_--Igneous
causes--Division of the subject--Distinct volcanic regions--Region
of the Andes--System of volcanoes extending from the Aleutian isles
to the Molucca and Sunda islands--Polynesian archipelago--