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 massive or unstratified rocks.  We shall see hereafter how the relative position of these two classes of rocks and their action upon each other enable 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 successfully 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.

[Illustration:  DIKES.]

Besides the stratified and massive 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 sandstone will be vitrified.  This is exactly what has been found to be the case, wherever the stratified rocks have been penetrated by the melted masses from beneath.  They have been themselves partially melted by the contact, and when they have cooled again, their stratification, though still perceptible, has been partly obliterated, and their substance changed.  Such effects may often be traced in dikes, which are only the cracks in rocks filled by materials poured into them at some