of pools, or go wholly dry, while at long intervals rains fill
their dusty beds with sudden raging torrents. Desert rivers
therefore periodically shorten and lengthen their courses,
withering back at times of drought for scores of miles, or even
for a hundred miles from the point reached by their waters during
seasons of rain.

THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams TRANSPORT
the waste of the land; they wear, or ERODE, their channels both on
bed and banks; and they DEPOSIT portions of their load from time
to time along their courses, finally laying it down in the sea.
Most of the work of streams is done at times of flood.

TRANSPORTATION

THE INVISIBLE LOAD OF STREAMS. Of the waste which a river
transports we may consider first the invisible load which it
carries in solution, supplied chiefly by springs but also in part
by the run-off and from the solution of the rocks of its bed. More
than half the dissolved solids in the water of the average river
consists of the carbonates of lime and magnesia; other substances
are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate
(Epsom salts), sodium chloride (common salt), and even silica, the
least soluble of the common rock-making minerals. The amount of
this invisible load is surprisingly large. The Mississippi, for
example, transports each year 113,000,000 tons of dissolved rock
to the Gulf.

THE VISIBLE LOAD OF STREAMS. This consists of the silt which the