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Updated: May 11, 2025
When geologists look at large and complicated phenomena, and then at the figures representing several million years, the two produce a totally different effect on the mind, and the figures are at once pronounced too small. In regard to subaerial denudation, Mr.
The least inclination given by M. E. de Beaumont of the upper surface of a lava-stream, namely 0 degrees 30 minutes, is that of the great subaerial eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows that it must have flowed down a mean inclination of less than 0 degrees 20 minutes.
Were it otherwise the subaerial denudation would be continually lessened by the diminution of the height and dimensions of the land exposed to waste.
PLAINS OF MARINE ABRASION. While subaerial denudation reduces the land to baselevel, the sea is sawing its edges to WAVE BASE, i.e. the lowest limit of the wave's effective wear. The widened rock bench forms when uplifted a plain of marine abrasion, which like the peneplain bevels across strata regardless of their various inclinations and various degrees of hardness.
Denudation defined. Its Amount more than equal to the entire Mass of Stratified Deposits in the Earth's Crust. Subaerial Denudation. Action of the Wind. Action of Running Water. Alluvium defined. Different Ages of Alluvium. Denuding Power of Rivers affected by Rise or Fall of Land. Littoral Denudation. Inland Sea-Cliffs. Escarpments. Submarine Denudation. Dogger-bank. Newfoundland Bank.
Subaerial denudation would not of itself lessen the area of the land, but would tend to fill up with sediment seas of moderate depth adjoining the coast.
Calcareous deposits and frondescent incrustations on the coast. Remarkable laminated beds, alternating with, and passing into, obsidian. Origin of obsidian. Lamination of volcanic rocks. The whole is volcanic, and, from the absence of proofs to the contrary, I believe of subaerial origin. The fundamental rock is everywhere of a pale colour, generally compact, and of a feldspathic nature.
The drifting of the withered leaves of autumn, or of the snows of winter over them, it really seems, would have done as much in that stretch of time. Then try to fancy the eternity it has taken the subaerial elements to cut thousands of feet through this hard Catskill sandstone!
Making due allowance for the quantity of matter removed by subaerial denudation in volcanic mountains of high antiquity, and for the grand explosions which are known to have caused truncation in active volcanoes, there is no reason for calling in the violent hypothesis of elevation craters to explain the structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of central France, Etna, or Vesuvius, all of which I have examined.
Considering the abundance of earthy lime near the summit of Red Hill, a volcanic cone six hundred feet in height, of subaerial growth, considering the intimate manner in which minute particles and large masses of scoriae are embedded in the masses of nearly pure lime, and on the other hand, the manner in which small kernels and streaks of the calcareous powder are included in solid pieces of the scoriae, considering, also, the similar occurrence of lime and scoriae within a stream of lava, also supposed, with good reason, to have been of modern subaerial origin, and to have flowed from a hill, where earthy lime also occurs: I think, considering these facts, there can be no doubt that the lime has been erupted, mingled with the molten lava.
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