3. Between these two difficulties the choice is embarrassing, and the decision must be unsatisfactory, except we can find some further ground of judgment. But perhaps this is not hopeless. We have hitherto referred to the evidence and analogies supplied by one science, namely, astronomy. But there are other sciences which give us information concerning the nature and history of the earth. From some of these, perhaps, we may obtain some knowledge of the place of the earth in the scheme of creation--how far it is, in its present condition, a thing unique, or only one thing among many like it. Any science which supplies us with evidence or information on this head, will give us aid in forming a judgment upon the question under our consideration. To such sciences, then, we will turn our attention. One science has employed itself in investigating the nature and history of the earth by an examination of the materials of which it is composed; namely, Geology. Let us call to mind some of the results at which this science has arrived. 4. A very little attention to what is going on among the materials of which the earth's surface is composed, suffices to show us that there are causes of change constantly and effectually at work. The earth's surface is composed of land and water, hills and valleys, rocks and rivers. But these features undergo change, and produce change in each other. The mountain-rivers cut deeper and deeper into the ravines in which they run; they break up the rocks over which they rush, use the fragments as implements of further destruction, pile them up in sloping mounds where the streams issue from the mountains, spread them over the plains, fill up lakes with sediment, push into the sea great deltas. The sea batters the cliffs and eats away the land, and again, forms banks and islands where there had been deep water. Volcanoes pour out streams of lava, which destroy the vegetation over which they flow, and which again, after a series of years, are themselves clothed with vegetation. Earthquakes throw down tracts of land beneath the sea, and elevate other tracts from the bottom of the ocean. These agencies are everywhere manifest; and though at a given moment, at a given spot, their effect may seem to us almost imperceptible, too insignificant to be taken account of, yet in a long course of years almost every place has undergone considerable changes. Rivers have altered their courses, lakes have become plains, coasts have been swept away or have become inland districts, rich valleys have been ravaged by watery or fiery deluges, the country has in some way or other assumed a new face. The present aspect of the earth is in some degree different from what it was a few thousand years ago.