There is no fact which the geological student is more constantly called upon to bear in mind than that of the potency of seemingly insignificant causes which continue in constant operation through long periods of time. Indeed these small and almost unnoticed agencies at work upon the earth's crust are often found, in the long ran, to produce far grander effects than those of which the action is much more striking and obvious. It is to the silent and imperceptible action of atmospheric moisture and frost that the disintegration of the solid rock-masses must be mainly ascribed; and the noisy cataract and ocean-billow produce effects which are quite insignificant compared with those which must be ascribed to the slight and almost unnoticed forces. Great masses of limestone are built up of the remains of microscopic organisms, while the larger and higher life-forms contribute but little to the great work of rock-building. [Sidenote: EFFECTS OF SLOW CONTINENTAL MOVEMENTS.] In the same way it is to the almost unnoticed action of the subterranean forces in raising some vast areas of the earth's crust, in depressing others, and in bringing about the development of mountain-chains between them, that we must ascribe a far more important part in the economy of our globe than to the more conspicuous but less constant action of volcanoes. A few simple considerations will serve to convince us, not only of the beneficial effects of the action of the subterranean energies within the earth's crust, but of the absolute necessity of the continued operation of those energies to the perpetuation of that set of conditions by which our planet is fitted to be the habitation of living beings. We have already referred to the prodigious effects which are constantly being produced around us by the action of the external forces at work upon the globe. The source of these external forces is found in the movements and changes which are ever going on within the aqueous and atmospheric media in which the globe is enveloped. The circulation of the air, influencing the circulation of the waters in the shape of clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking up of even the hardest rock-masses, and the continual removal of their disintegrated fragments from higher to lower levels. This work goes on with more or less regularity over every part of the land raised above the level of the ocean, but the rate of destruction in the higher regions of the atmosphere is far more rapid than at lower levels. Hence the circulating air and water of the globe are found to be continually acting as levellers of the land-masses of the earth. It is by no means a difficult task to calculate the approximate rate at which the various continents and islands are being levelled down, and such calculations prove that in a very few millions of years the existing forces operating upon the earth's surface would reduce the whole of the land-masses to the level of the ocean.