It is a fact, which is capable of mathematical demonstration, that by the action of these denuding forces the surface of all the lands of the globe is being gradually but surely lowered; and this takes place at such a rate that in a few millions of years the whole of the existing continents must be washed away and their materials distributed over the beds of the oceans. [Sidenote: NECESSITY FOR COMPENSATING AGENCIES.] It is evident that there exists some agency by which this levelling action of the denuding forces of the globe is compensated; and a little consideration will show that such compensating agency is found in the subterranean forces ever at work within the earth's crust. The effects of these subterranean forces which most powerfully arrest our attention are volcanic outbursts and earthquake shocks, but a careful study of the subject proves that these are by no means the most important of the results of the action of such forces. Exact observation has proved that almost every part of the earth's surface is either rising or falling, and the striking and destructive phenomena of volcanoes and earthquakes probably bear only the same relation to those grand and useful actions of the subterranean forces, which floods do to the system of circulating waters, and hurricanes to the system of moving air-currents. If we ride in a well-appointed carriage with good springs, upon a railway which is in excellent order, the movement is almost imperceptible to us; and the rate of speed may be increased indefinitely, without making itself apparent to our senses. The smallest impediment to the evenness of the movement--such as that produced by a small object placed upon the rails--at once makes itself felt by a violent jar and vibration. How perfectly insensible we may be of the grandest and most rapid movements is taught us by the facts demonstrated by the astronomer. By the earth's daily rotation, we are borne along at a rate which in some places amounts to over 1,000 miles an hour; and by its annual revolution we are every hour transported through a distance of 70,000 miles; yet concerning the fact and direction of these movements we are wholly unconscious. In the case both of the railway train and of our planet, we can only establish the reality of the movement, and its direction and rate, by means of observations upon external objects, which appear to us to have a movement in the opposite direction. In the same way we can only establish the fact of the movement of portions of the earth's crust by noticing the changing positions of parts of the earth's surface in relation to the constant level of the ocean. When this is done we find abundant proof that while some parts of the earth's crust are rising, others are as undoubtedly undergoing depression. [Sidenote: POTENCY OF THE SUBTERRANEAN FORCES.]