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Updated: May 11, 2025
We are best acquainted with the effects of eruptions above water, or those called subaerial or supramarine; yet the products even of these are arranged in so many ways that their interpretation has given rise to a variety of contradictory opinions, some of which will have to be considered in this chapter. Part of the chain of extinct volcanoes called the Monts Dome, Auvergne.
The widening of the canon was doubtless a slower process than the downward cutting. But, as I have said, the downward cutting would tend to check itself from age to age, while the widening process would go steadily forward. Hence, when we look into the great abyss, we have only to remember the enormous length of time that the aerial and subaerial forces have been at work to account for it.
Part of the action of the waves between high and low watermark must be included in subaerial denudation, more especially as the undermining of cliffs by the waves is facilitated by land-springs, and these often lead to the sliding down of great masses of land into the sea.
This bed is remarkable from containing many globular and pear-shaped, externally rusty balls, from the size of an apple to a man's head, of very tough, slate-coloured porphyry, with imperfect crystals of feldspar: in shape these balls do not resemble pebbles, AND I BELIEVE THAT THEY ARE SUBAQUEOUS VOLCANIC BOMBS; they differ from SUBAERIAL bombs only in not being vesicular.
When thus impressed with the slow rate at which the land is worn away through subaerial and littoral action, it is good, in order to appreciate the past duration of time, to consider, on the one hand, the masses of rock which have been removed over many extensive areas, and on the other hand the thickness of our sedimentary formations.
Denudation may be divided into subaerial, or the action of wind, rain, and rivers; and submarine, or that effected by the waves of the sea, and its tides and currents. With the operation of the first of these we are best acquainted, and it may be well to give it our first attention.
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 left projecting.
These rocks form the mountains called the Arans and the Arenigs; numerous greenstones are associated with them, which are intrusive, although they often run in the lines of bedding for a space. "Much of the ash," says Professor Ramsay, "seems to have been subaerial.
Hence it seems that along this coast, as at Bermuda and at Keeling Atoll, submarine and subaerial deposits are contemporaneously in process of formation, from the disintegration of marine organic bodies. The extent of these deposits, considering their origin, is very striking; and they can be compared in this respect only with the great coral-reefs of the Indian and Pacific Oceans.
We have, however, recently learned from the observations of Ramsay, in the van of many excellent observers of Jukes, Geikie, Croll and others, that subaerial degradation is a much more important agency than coast-action, or the power of the waves.
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