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.

The rocks formed under the influence of heat are called, in geological language, the Igneous, or, as some naturalists have named them, the Plutonic rocks, alluding to their fiery origin, while the others have been called Aqueous or Neptunic rocks, in reference to their origin under the agency of water.  A simpler term, however, quite as distinctive, and more descriptive of their structure, is that of the stratified and unstratified or massive rocks.  We shall see hereafter how the relative position of these two kinds of rocks and their action upon each other enables us to determine the chronology of the earth, to compare the age of her mountains, and if we have no standard by which to estimate the positive duration of her continents, to say at least which was the first-born among them, and how their characteristic features have been successively worked out.  I am aware that many of these inferences, drawn from what is called “the geological record,” must seem to be the work of the imagination.  In a certain sense this is true,—­for imagination, chastened by correct observation, is our best guide in the study of Nature.  We are too apt to associate the exercise of this faculty with works of fiction, while it is in fact the keenest detective of truth.

Beside the stratified and unstratified rocks, there is still a third set, produced by the contact of these two, and called, in consequence of the changes thus brought about, the Metamorphic rocks.  The effect of heat upon clay is to bake it into slate; limestone under the influence of heat becomes quick-lime, or if subjected afterwards to the action of water, it is changed to mortar; sand under the same agency is changed to a coarse kind of glass.  Suppose, then, that a volcanic eruption takes place in a region of the earth’s surface where successive layers of limestone, of clay, and of sandstone have been previously deposited by the action of water.  If such an eruption has force enough to break through these beds, the hot, melted masses will pour out through the rent, flow over its edges, and fill all the lesser cracks and fissures produced by such a disturbance.  What will be the effect upon the stratified rocks?  Wherever these liquid masses, melted by a heat more intense than can be produced by any artificial means, have flowed over them or cooled in immediate contact with them, the clays will be changed to slate, the limestone will have assumed a character more like marble, while the sandstones will be vitrified.  This is