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Such rapid changes from complete saturation to sudden heat are trying to the hardest rocks, and at Hardraw, close at hand, there is a still more palpable process of denudation in active operation. Such a morning as this is quite ideal for seeing the remarkable waterfall known as Hardraw Scar or Force.
Some, as we shall see, are due to upwarps or to fractures of the crust; some are piles of volcanic material; some are swellings caused by the intrusion of molten matter beneath the surface; some are the relicts left after the long denudation of high plateaus.
I remember having been much struck with the evidence of denudation, when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean.
Formed of rough and crystalline products of the basic fire of earth, they hold high up in their recesses coral beds once under the sea, and lava in many shapes, tokens of the island's rise from the slime, and of mammoth craters now almost entirely obliterated by denudation the denudation which made the level land as fertile as any on earth, and the suitable habitation of the most leisurely and magnificent human animals of history.
Whether the loftiest waves were the result of one great convulsion, or of a long-continued succession of small ones, the effect would be the same, namely, that the strata over those points at which the granite penetrated the highest, would be the most dislocated, and the most exposed to wear during denudation.
First, let us take the case of the bed of the sea long remaining at a stationary level: under these circumstances it is evident that CONCHIFEROUS strata can accumulate only to the same thickness with the depth at which the shells can live; on gently inclined coasts alone can they accumulate to any considerable width; and from the want of superincumbent pressure, it is probable that the sedimentary matter will seldom be much consolidated: such formations have no very good chance, when in the course of time they are upraised, of long resisting the powers of denudation.
It is perhaps, on this account, the most striking of the numerous examples exhibited throughout the West of Scotland and the North-east of Ireland of the enormous amount of denudation to which these districts have been subjected since the extinction of the volcanic fires; and this at a period to which we cannot assign a date more ancient than that of the Pliocene.
The origin of its component mineral matter must be sought in the denudation of the Himalayas within a very recent geological period. The contrast between the fertility of the alluvium and the sterility of the protruded quartzy rocks is very striking, cultivation running up to these fields of stones, and suddenly stopping.
"'Not proven' is the verdict which most geologists would return." "The confidence with which Croll's theory was first received has been a good deal shaken." "We have to fall back, therefore, on the geological evidence of deposition and denudation ... in any attempt to decide between the 200,000 years of Lyell and the 20,000 years of Prestwich."
While one region of the Earth's crust, continuing for a vast epoch above the surface of the ocean, bears record of no changes save those resulting from denudation; another region of the Earth's crust gives proof of sundry changes of level, with their several resulting masses of stratified detritus.
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