Note 4 — THE CIRCLE OF THE SCIENCES.

“Carboniferous period.” Coal age. By careful study it has been found that in the progress of the earth’s development a number of great ages have existed—each distinguished from the others by some marked change. That of coal plants is placed by geologists as the fourth age, counting upward from the lowest formation. It was remarkable for the alternate low elevation of the land above the sea level, and its submergences; and also for the luxuriant growth of vegetation, which, under the great pressure and heat to which it was subjected while the surface was submerged, was changed into coal.