We shall be able to form some idea of the vastness of the effects produced by the subterranean forces, by a very simple consideration. It is certain that during the enormous periods of time of which the records have been discovered by the geologist, there have always been continents and oceans upon the earth's surface, just as at present, and it is almost equally certain that the proportions of the earth's surface occupied by land and water respectively, have not varied very widely from those which now prevail. But, at the same time, it is an equally well-established bet that the denuding forces ever at work upon the earth's surface would have been competent to the removal of existing continents many times over, in the vast periods covered by geological records. Hence we are driven to conclude that the subterranean movements have in past times entirely compensated for the waste produced by the denuding forces ever at work upon our globe. But this is not all. The subterranean forces not only produce upheaval; in a great many cases the evidences of subsidence are as clear and conclusive as are those of upheaval in others. Hence we are driven to conclude that the forces producing upheaval of portions of the earth's crust are sufficient, not only to balance those producing subsidence, but also to compensate for the destructive action of denuding agents upon the land-masses of the globe. It is only by a careful and attentive study and calculation of the effects produced by the denuding agents at work all around us, aided by an examination of the enormous thicknesses of strata formed by the action of such causes during past geological times, that we are able to form any idea of the reality and vastness of the agents of change which are ever operating to modify the earth's external features. When we have clearly realised the grand effects produced on the surface of the globe by these external forces, through the action of its investing atmosphere and circulating waters, then, and only then, shall we be in a position to estimate the far greater effects resulting from the internal forces, of which the most striking, but not the most important, results are seen in the production of volcanic eruptions and earthquake-shocks.