rising of vapors, their condensation into clouds, the falling of rains, the gathering of waters upon its surface.  Water is a very active agent of destruction, but it works over again the materials it pulls down or wears away, and builds them up anew in other forms.  As soon as an ocean washed over the consolidated crust of the globe, it would begin to abrade the surfaces upon which it moved, gradually loosening and detaching materials, to deposit them again as sand or mud or pebbles at its bottom in successive layers, one above another.  Thus, in analyzing the crust of the globe, we find at once two kinds of rocks, the respective work of fire and water:  the first poured out from the furnaces within, and cooling, as one may see any mass of metal cool that is poured out from a smelting-furnace to-day, in solid crystalline masses, without any division into separate layers or leaves; and the latter in successive beds, one over another, the heavier materials below, the lighter above, or sometimes in alternate layers, as special causes may have determined successive deposits of lighter or heavier materials at some given spot.

There were many well-fought battles between geologists before it was understood that these two elements had been equally active in building up the crust of the earth.  The ground was hotly contested by the disciples of the two geological schools, one of which held that the solid envelope of the earth was exclusively due to the influence of fire, while the other insisted that it had been accumulated wholly under the agency of water.  This difference of opinion grew up very naturally; for the great leaders of the two schools lived in different localities, and pursued their investigations over regions where the geological phenomena were of an entirely opposite character,—­the one exhibiting the effect of volcanic eruptions, the other that of stratified deposits.  It was the old story of the two knights on opposite sides of the shield, one swearing that it was made of gold, the other that it was made of silver; and almost killing each other before they discovered that it was made of both.  So prone are men to hug their theories and shut their eyes to any antagonistic facts, that it is related of Werner, the great leader of the Aqueous school, that he was actually on his way to see a geological locality of especial interest, but, being told that it confirmed the views of his opponents, he turned round and went home again, refusing to see what might force him to change his opinions.  If the rocks did not confirm his theory, so much the worse for the rocks,—­he would none of them.  At last it was found that the two great chemists, fire and water, had worked together in the vast laboratory of the globe, and since then scientific men have decided to work together also; and if they still have a passage at arms occasionally over some doubtful point, yet the results of their investigations are ever drawing them nearer to each other,—­since men who study truth, when they reach their goal, must always meet at last on common ground.