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of intensity in consequence of the absolutely reduced down-stream impulse; also (d) material increase of friction with the augmentation of its principal factor, and (e) less material diminution of friction in consequence of the reduced impulse; and finally (f), direct diminution of effectiveness with the absolute decrease of impulse, (g) indirect diminution of effectiveness in consequence of the relative decrease of the same factor, and (h) direct but slight increase of effectiveness in virtue of the operation of the obscure factor of rock-crushing and pressure-liquefaction; or, summarily, increase in intensity, slight increase in friction, and decrease in effectiveness.

Now, in view of the obscure and antagonistic though inter-

Fig. 4.

dependent relations involved, it is evident that without exhaustive quantitative investigation (impossible in the present absence of knowledge concerning friction between ice and other substances) it cannot be determined in the ordinary case whether the disposition will be to erode the more rapidly where weight increases at the expense of declivity, or where the reverse occurs; but it appears quite certain that where the surface declivity materially exceeds that at the base, and where, accordingly, the impulse is not reduced proportionally to the declivity of the channel, erosion must progressively increase with the weight. If so, the tendency of glaciers must be to cumulatively intensify the irregularities in gradient normal to water-cut cañons.

But corrasion and transportation in any part of a glacier-bed are limited directly by flow of ice and indirectly by coincident