t all plant life, and covering the layers of peat with
beds of sand or mud. When the water went down, other forests took
possession, and a new coal-bed was started. It is plainly seen that
flooding often put an end to coal formation. Fifteen seams of coal, one
above another, is the greatest number that have been found. The veins
vary from one inch to forty feet in thickness. These are separated by
layers of sandstone or shale, which accumulated as sediment, covering
the stumps of dead tree ferns and other growths, and preserving them as
fossils to tell the story of those bygone ages as plainly as any other
record could have done.

Fresh-water animals succeeded those of salt water in the swamps that
formed the coal measures. Overhead, the first insects flitted among the
branches of the tree ferns. Dragon-flies darted above the surface and
dipped in water as they do to-day. Spiders, scorpions, and cockroaches,
all air-breathing insects, were represented, but none of the higher,
nectar-loving insects, like flies and bees and butterflies, were there.
Flowering plants had not yet appeared on the earth. Snakelike
amphibians, some fishlike, some lizard-like, and huge crocodilian forms
appeared for the first time. These air-breathing swamp-dwellers could
not have lived in salt water.

Fresh-water molluscs and land shells appear for the first time as
fossils in the rocks of the coal measures. On the shores of the ocean,
the rocks of this period show that trilobites, horseshoe crabs, and
fishes still lived in vast numbers, and corals continued to form
limestone. The old types of marine animals changed gradually, but the
coal measures show strikingly different fossils. These rocks bear the
first record of fresh-water and land animals.

THE MOST USEFUL METAL

It is fortunate for us all that, out of the half-dozen so-called useful
metals, iron, which is the most useful of them all to the human race,
should be also the most plentiful and the cheapest. Aluminum is abundant
in the common clay and soil under our feet. But separating it is still
an expensive process; so that this metal is not commercially so
plentiful as iron is, nor is it cheap.

All we know of the earth's substance is based on studies of the
superficial part of its crust, a mere film compared with the eight
thousand miles of its diameter. Nobody knows what the core of the
earth--the great globe under this surface film--is made of; but we know
that it is o