| → the coast, 1869 1872 |
| lines of sea-coast, 1859 1860 1861 1866 |
|
| → good evidence 1860 1861 1866 1869 1872 |
| reason to believe 1859 |
|
| → after being reduced 1869 1872 |
| reduced 1859 1860 1861 1866 |
|
| → many excellent observers— 1872 |
| excellent observers, 1869 |
|
| → OMIT 1872 |
| by every one 1869 |
|
|
It is good to wander along
→the coast,
when formed of moderately hard rocks, and mark the process of degradation. The tides in most cases reach the cliffs only for a short time twice a day, and the waves eat into them only when they are charged with sand or pebbles; for there is
→good evidence
that pure water
nothing in wearing away rock. At last the base of the cliff is undermined, huge fragments fall down, and
remaining fixed, have to be worn
atom by atom,
→after being reduced
in
they can be rolled about by the waves, and then
more quickly ground into pebbles, sand, or mud. But how often do we see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded and how seldom they are rolled about! Moreover, if we follow for a few miles any line of rocky cliff, which is undergoing degradation, we find that it is only here and there, along a short length or round a promontory, that the cliffs are at the present time suffering. The appearance of the surface and the vegetation show that
years have elapsed since the waters washed their base. |
|
| We have, however, recently learnt from the observations of Ramsay, in the van of
→many excellent observers—
of Jukes, Geikie, Croll, and others, that subaerial
is a much more important agency than coast-action, or the power of the waves. The whole surface of the land is exposed to the chemical action of the air and of the rain-water with its dissolved carbonic acid, and in colder countries to frost; the disintegrated matter is carried down even gentle slopes during heavy rain, and to a greater extent than might be supposed, especially in arid districts, by the wind; it is then transported by the streams and rivers, which when rapid deepen their channels, and triturate the fragments. On a rainy day, even in a gently undulating country, we see the effects of subaerial degradation in the muddy rills which flow down
slope. Messrs. Ramsay and Whitaker have shown, and the observation is a most striking one, that the great lines of escarpment in the Wealden district and those ranging across England, which formerly were looked at
→OMIT
as ancient sea-coasts, cannot have been thus formed, for each line is composed of one and the same formation, whilst our
sea-cliffs are everywhere formed by the intersection of various formations. This being the case, we are compelled to admit that the escarpments owe their origin in chief part to the rocks of which they are composed having resisted subaerial denudation better than the surrounding surface; this surface consequently has been gradually lowered, with the lines of harder rock
|
