Twenty miles is only one four-hundredth part of the earth’s diameter—­a mere thin shell over a massive globe.  If the earth were brought down in size to an ordinary large school globe, a piece of rough brown paper covering it might well represent the thickness of this earth-crust, with which the science of geology has to do.  And the whole of the globe, this earth of ours, is but one tiny planet in the great Solar System.  And the centre of that Solar System, the blazing sun, though equal in size to more than a million earths, is yet himself but one star amid millions of twinkling stars, scattered broadcast through the universe.  So it would seem at first sight that the field of geology is a small field compared with that of astronomy....

With regard to the great bulk of the globe little can be said.  Very probably it is formed through and through of the same materials as the crust.  This we do not know.  Neither can we tell, even if it be so formed, whether the said materials are solid and cold like the outside crust, or whether they are liquid with heat.  The belief has been long and widely held that the whole inside of the earth is one vast lake or furnace of melted fiery-hot material, with only a thin cooled crust covering it.  Some in the present day are inclined to question this, and hold rather that the earth is solid and cold throughout, though with large lakes of liquid fire here and there, under or in her crust, from which our volcanoes are fed....

The materials of which the crust is made are many and various; yet, generally speaking, they may all be classed under one simple word, and that word is—­Rock.

It must be understood that, when we talk of rock in this geological sense, we do not only mean hard and solid stone, as in common conversation.  Rock may be changed by heat into a liquid or “molten” state, as ice is changed by heat to water.  Liquid rock may be changed by yet greater heat to vapor, as water is changed to steam, only we have in a common way no such heat at command as would be needed to effect this.  Rock may be hard or soft.  Rock maybe chalky, clayey, or sandy.  Rock may be so close-grained that strong force is needed to break it; or it may be so porous—­so full of tiny holes—­that water will drain through it; or it may be crushed and crumbled into loose grains, among which you can pass your fingers.

The cliffs above our beaches are rock; the sand upon our seashore is rock; the clay used in brick-making is rock; the limestone of the quarry is rock; the marble of which our mantel-pieces are made is rock.  The soft sandstone of South Devon, and the hard granite of the north of Scotland, are alike rock.  The pebbles in the road are rock; the very mould in our gardens is largely composed of crumbled rock.  So the word in its geological sense is a word of wide meaning.

Now the business of the geologist is to read the history of the past in these rocks of which the earth’s crust is made.  This may seem a singular thing to do, and I can assure you it is not an easy task.