Wonders of Earth, Sea and Sky | Page 4

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.
For, to begin with, the history itself is written in a strange language, a language which man is only just beginning to spell out and understand. And this is only half the difficulty with which we have to struggle.
If a large and learned book were put before you and you were set to read it through, you would perhaps, have no insurmountable difficulty, with patience and perseverance, in mastering its meaning.
But how if the book were first chopped up into pieces, if part of it were flung away out of reach, if part of it were crushed into a pulp, if the numbering of the pages were in many places lost, if the whole were mixed up in confusion, and if then you were desired to sort, and arrange, and study the volume?
Picture to yourself what sort of a task this would be, and you will have some idea of the labors of the patient geologist.
Rocks may be divided into several kinds or classes. For the present moment it will be enough to consider the two grand divisions--Stratified rocks and Unstratified rocks.
Unstratified rocks are those which were once, at a time more or less distant, in a melted state from intense heat, and which have since cooled into a half crystallized state; much the same as water, when growing colder, cools and crystallizes into ice. Strictly speaking, ice is rock, just as much as granite and sandstone are rock. Water itself is of the nature of rock, only as we commonly know it in the liquid state we do not commonly call it so.
[Illustration: UNSTRATIFIED ROCK.--A VOLCANIC BLOCK.]
"Crystallization" means those particular forms or shapes in which the particles of a liquid arrange themselves, as that liquid hardens into a solid--in other words, as it freezes. Granite, iron, marble, are frozen substances, just as truly as ice is a frozen substance; for with greater heat they would all become liquid like water. When a liquid freezes, there are always crystals formed, though these are not always visible