lly sealed
under the great earth strata. The sun gathered up the material and set
the forces in play which made the chemical combinations of the various
elements in nature that enter into vegetable growth.

After the lapse of untold ages of time these great beds of stored-up
sun-energy were discovered by man and their contents are dragged out to
the earth's surface, to warm our houses, to drive the machinery of our
factories, to send the locomotives flying across the continents and the
steamships over the oceans. So important has this article become that if
any one nation could control the output it would be able to paralyze all
the navies and the manufacturing of the world.

If the coal of the world should become exhausted we should be confronted
with a great problem. Fortunately for us, this is a problem that will
have to be solved by the people of some future age, as the growth of
wood will scarcely keep pace with the consumption of fuel. By that time
the genius of man will have devised an economical means of storing the
energy of the sunbeams directly for purposes of heat, light, and power.

CHAPTER IV.

SLATE AND SHALE.

Slate is one of the great commercial products of the world. As far back
as the year 1877 the output of slate was not less than 1,000,000 tons
per annum. The chief use to which slate is put is for covering
buildings, and for this purpose it is better than any other known
material. It is also used in the construction of billiard tables and for
writing-slates; these latter uses are very insignificant as compared to
its use in architecture. Slate, like building-stone and limestone, is
quarried from the earth's crust and is found in the strata close above
the Metamorphic rocks, near the beginning of what is called the Primary,
or Paleozoic period. As compared with the coal formations it is very,
very old.

There are different substances called slate that are not slate in the
scientific use of that word. In general all stone formations are called
slates that split up into thin layers. But the true slate is a special
material which is formed by special processes of nature. The difference
between slate and shale, for instance, is not one of ingredients, but of
the process by which the ingredients are put together. All of the
sedimentary rocks are formed by a deposit of sediment from the water on
the bottom of the ocean. At one period the floods have brought down a
certain kind of material in