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Updated: June 3, 2025
In order to bring them into view, or within reach of the miner, a greater amount of upheaval and denudation must take place in proportion as they have lain deeper when first formed and filled.
To begin with the oldest set of rocks, No. 1; they consist chiefly of hornblendic gneiss, and in the neighbouring Hebrides form whole islands, attaining a thickness of thousands of feet, although they have suffered such contortions and denudation that they seldom rise more than a few hundred feet above the sea-level. Laurentian gneiss. 2.
The student may refer to Mr. Hull's measurement of faults, observed in the Lancashire coal-field, where the vertical displacement has amounted to thousands of feet, and yet where all the superficial inequalities which must have resulted from such movements have been obliterated by subsequent denudation.
We may fairly call them denudation mountains. At this stage of the inquiry we have to account for two remarkable phenomena.
In both cases, denudation carried on throughout perhaps the Pliocene and Post-Pliocene periods down to the present day has had the effect of demolishing the original craters; so that what we now observe as forming these ranges are the consolidated columns of original molten matter which filled the throats of the old volcanoes, or the sheets of lava which were extruded from them, but are now probably much reduced in size and extent.
During the slow upheaval and denudation of the lodes, they have been subjected to various chemical, hydro-thermal, and aqueous agencies, by which many of their contents have been re-arranged and re-formed, new minerals have been brought in by percolation of water from the surrounding rocks, and possibly some of the original contents have been carried out by mineral springs rising through the lines of fissures which are not completely sealed by the igneous injection, as the contraction of the molten matter in cooling has left cracks and crevices through which water readily passes.
The raised reef is here preserved, but over the portion of the island immediately fronting Cebu it has been removed by denudation, with the exception of a few pillar-like blocks which remain, and which are conspicuous from the anchorage. The surface is scooped out into irregular basins and sharp projecting pinnacles and covered in all directions with mud, resulting from the denudation.
Second, the wearing away and reduction of these formations to their present level, by a denudation, more extensive than any thus far recorded in the annals of geology, which has given rise to all the most prominent hills and mountain chains along the northern bank of the river. Before seeking an explanation of these facts, let us look at the third and uppermost deposit.
A. Tylor had adduced important evidence on sub-aerial denudation, by showing that the amount of matter brought down by rivers must infallibly lower the level of their drainage basins by many feet in no immense lapse of time. This line of argument has since been followed up in the most interesting manner by Archibald Geikie, Croll and others, in a series of valuable memoirs.
Hence I conclude, that it is infinitely more probable that the strata were accumulated under the sea: the vast amount of denudation, moreover, which this range has suffered, as shown by the wide valleys, by the exposure of the very trees and by other appearances, could have been effected, I conceive, only by the long-continued action of the sea; and this shows that the range was either upheaved from under the sea, or subsequently let down into it.
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