Secondly.—­Plutonic Rocks, such as granite.  These have been slowly cooled deep down in the earth under heavy pressure.

There is also a class of rocks, called metamorphic rocks, including some kinds of marble.  These are, strictly speaking, crystalline rocks, and yet they are arranged in something like layers.  The word “metamorphic” simply means “transformed.”  They are believed to have been once stratified rocks, perhaps containing often the remains of animals; but intense heat has later transformed them into crystalline rocks, and the animal remains have almost or quite vanished.

[Illustration:  LAVA-STREAM ON VESUVIUS.]

Just as the different kinds of Stratified Rocks are often called Aqueous Rocks, or rocks formed by the action of water—­so these different kinds of Unstratified Rocks are often called Igneous Rocks, or rocks formed by the action of fire—­the name being taken from the Latin word for fire.  The Metamorphic Rocks are sometimes described as “Aqueo-igneous,” since both water and fire helped in the forming of them.

It was at one time believed, as a matter of certainty, that granite and such rocks belonged to a period much farther back than the periods of the stratified rocks.  That is to say, it was supposed that fire-action had come first and water-action second; that the fire-made rocks were all formed in very early ages, and that only water-made rocks still continued to be formed.  So the name of Primary Rocks, or First Rocks, was given to the granites and other such rocks, and the name of Secondary Rocks to all water-built rocks; while those of the third class were called Transition Rocks, because they seemed to be a kind of link or stepping-stone in the change from the First to the Second Rocks.

The chief reason for the general belief that fire-built rocks were older than water-built ones was, that the former are as a rule found to lie lower than the latter.  They form, as it were, the basement of the building, while the top-stories are made of water-built rocks.

Many still believe that there is much truth in the thought.  It is most probable, so far as we are able to judge, that the first-formed crust of rocks all over the earth was of cooled and crystallized material.  As these rocks were crumbled and wasted by the ocean, materials would have been supplied for the building-up of rocks, layer upon layer.

But this is conjecture.  We cannot know with any certainty the course of events so far back in the past.  And geologists are now able to state with tolerable confidence that, however old many of the granites may be, yet a large amount of the fire-built rocks are no older than the water-built rocks which lie over them.

So by many geologists the names of Primary, Transition, and Secondary Formations are pretty well given up.  It has been proposed to give instead to the crystallized rocks of all kinds the name of Underlying Rocks (Hypogene Rocks).