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Published online by Cambridge University Press: 01 May 2009
The importance of experiments and investigations such as those recorded by the Rev. J. D. La Touche, in the Geological Magazine for April, can hardly be over-estimated from a geological point of view, and, if carried on in numerous districts and for a sufficient length of time, would furnish data with which, and a basis from which, future calculations as to the rate of denudation at present going on over the land surface of the globe might be esti- mated. For it cannot be denied that at present there is a laxness in the method of computation as regards the rate and amount of degradation both for past and present epochs which ill accords with the ultimate end and aim of all scientific pursuits, viz., truth. Each one taking a rate which seems most to confirm his own particular tenets, as, for instance, the different rates at which the river Niagara has out its way back from Queenstown Heights to the present situa- tion of the Tails. It has been taken as three feet, one foot, six inches, and one inch per annum, by different computators. Now I do not say that investigations on the river action of the period in which we live will clear up all such anomalies, because these obser- vations will be of more service in estimating the future rate of the wearing down of the materials composing the land than that of the past, as there would always be the difficulty of ascertaining with precision the volume and velocity of the rivers, and the different elevations at which the land was situated at the particular era under consideration; still it would be a fulcrum on which we might rest the lever of analogy, and raise many questions at present resting in obscurity and doubt into light and certainty.
page 268 note 1 The river that flows from Lough Cooter County Galway, at the place now called the “Devil's punch bowl,” a few miles southward of Gort, has gradually eaten into an accumulation of Boulder-clay drift, until the drift cliff over the vent, a natural filter, is about 100 feet high. This drift is similar to the general Boulder-clay drift of the central plain of Ireland, being boulders and fragments of Carboniferous limestone with some of sandstone, contained in a clayey, slightly sandy matrix Yearly this stream during floods, carries away a mass of the drift, yet when the water is low, few or none of the boulders and fragments that apparently ought to have been left as a residue of the drift can be seen, the river bed containing scarcely anything else but angular cherty gravel. Some parts of the blocks and fragments may have disappeared by abrasion, and the limestone may possibly dissolve and go away in solution, but how have theydisappeared so rapidly, and what has become of the pieces of grits and sandstones? The latter are not very numerous, yet there are many of them, and how have they disappeared, as they could not have gone through the niter, and they could scarcely have dissolved and gone off in solution?
page 269 note 1 If we suppose that r is the rainfall, d the discharge, and s the sediment, during any part of the period when the experiments were conducted; r 1 the rainfall, d 1 the discharge, and s 1 the sediment, at any future equal length of time, then—
page 271 note 1 and 5 Civil Engineering, Professor Rankine.
page 271 note 2 and 4 Hydraulics, M. Du Bust.
page 271 note 3 Elements of Geology, Sir Charles Lyell.
page 271 note 6 Principles, Sir C. Lyell.