o a point where, in the
manner of the continental shelf, the bottom descends steeply into deep
water.

It is the custom of naturalists to divide the lower section of river
deposits--that part of the accumulation which is near the sea--from
the other alluvial plains, terming the lower portion the delta. The
word originally came into use to describe that part of the alluvium
accumulated by the Nile near its mouth, which forms a fertile
territory shaped somewhat like the fourth letter of the Greek
alphabet. Although the definition is good in the Egyptian instance,
and has a certain use elsewhere, we best regard all the detritus in a
river valley which is in the state of repose along the stream to its
utmost branches as forming one great whole. It is, indeed, one of the
most united of the large features which the earth exhibits. The
student should consider it as a continuous inclined plane of
diminishing slope, extending from the base of the torrents to the
sea, and of course ramifying into the several branches of the river
system. He should further bear in mind the fact that it is a vast
laboratory where rock material is brought into the soluble state for
delivery to the seas.

The diversity in the form of river valleys is exceedingly great.
Almost all the variety of the landscape is due to this impress of
water action which has operated on the surface in past ages. When
first elevated above the sea, the surface of the land is but little
varied; at this stage in the development the rivers have but shallow
valleys, which generally cut rather straight away over the plain
toward the sea. It is when the surface has been uplifted to a
considerable height, and especially when, as is usually the case, this
uplifting action has been associated with mountain-building, that
valleys take on their accented and picturesque form. The reason for
this is easily perceived: it lies in the fact that the rocks over
which the stream flows are guided in the cutting which they effect by
the diversities of hardness in the strata that they encounter. The
work which it does is performed by the hard substances that are
impelled by the current, principally by the sand and pebbles. These
materials, driven along by the stream, become eroding tools of very
considerable energy. As will be seen when we shortly come to describe
waterfalls, the potholes formed at those points afford excellent
evidence as to the capacity of stream-impelled bits of stone t