trata at Kinghorn,
in Fife, showing stratified beds (limestone and shales) surmounted
by an unstratified mass of trap. (Original.)]

As regards the origin of the Sedimentary Rocks, they are for
the most part "derivative" rocks, being derived from the wear
and tear of pre-existent rocks. Sometimes, however, they owe
their origin to chemical or vital action, when they would more
properly be spoken of simply as Aqueous Rocks. As to their mode
of deposition, we are enabled to infer that the materials which
compose them have formerly been spread out by the action of water,
from what we see going on every day at the mouths of our great
rivers, and on a smaller scale wherever there is running water.
Every stream, where it runs into a lake or into the sea, carries
with it a burden of mud, sand, and rounded pebbles, derived from
the waste of the rocks which form its bed and banks. When these
materials cease to be impelled by the force of the moving water,
they sink to the bottom, the heaviest pebbles, of course, sinking
first, the smaller pebbles and sand next, and the finest mud
last. Ultimately, therefore, as might have been inferred upon
theoretical grounds, and as is proved by practical experience,
every lake becomes a receptacle for a series of stratified rocks
produced by the streams flowing into it. These deposits may vary
in different parts of the lake, according as one stream brought
down one kind of material and another stream contributed another
material; but in all cases the materials will bear ample evidence
that they were produced, sorted, and deposited by running water.
The finer beds of clay or sand will all be arranged in thicker or
thinner layers or laminae; and if there are any beds of pebbles
these will all be rounded or smooth, just like the water-worn
pebbles of any brook-course. In all probability, also, we should
find in some of the beds the remains of fresh-water shells or
plants or other organisms which inhabited the lake at the time
these beds were being deposited.

In the same way large rivers--such as the Ganges or
Mississippi--deposit all the materials which they bring down
at their mouths, forming in this way their "deltas." Whenever
such a delta is cut through, either by man or by some channel of
the river altering its course, we find that it is composed of a
succession of horizontal layers or strata of sand or mud, varying
in mineral composition, in structure, or in grain, according to
the n