The Story of Slate - 1923 - Page 2
The Story of Slate
To understand slate properly one must know something of its origin and must be able to follow in some measure the slow but powerful geologic forces that have shaped it into its present form.
The natural history of slate falls into two great periods, each period covering thousands and possibly millions of years.
The first period has to do with the origin of the mineral particles of which slate is formed.
All rocks slowly wear away and disintegrate through the action of frost and water.
Sometimes fiction writers, orators or poets speak of the everlasting hills — but no hills are everlasting — the rocks that form them are slowly but continuously changing into new forms that are carried away and are built up into other rocks.
Many minerals, particularly the feldspars, slowly change into clay and this clay is washed down by the rain, carried by streams into rivers, and by rivers to the sea.
Clay particles are very small and light, so are carried far from shore and deposited in uniform beds on the ocean floor.
Such clay beds are the raw materials from which slate is produced by the forces of nature.
The second great period comprises the centuries during which the unseen forces of nature convert the clay into slate as we now find it in the earth.
The clay beds are first compressed into a firm mass known as shale.
The shales may later be intensely folded and squeezed.
It is generally assumed that the earth is slowly cooling, and as it cools, it shrinks.
This shrinkage causes a wrinkling or folding of the surface rocks just as the skin of an apple wrinkles when it is baked.
When the shales are caught in these folds they are squeezed with terrific force and under high temperature.
The forces are so great that in some mysterious manner the clay is converted into