Among the astounding discoveries of modern science is that of the immense periods which have passed in the gradual formation of our earth. So vast were the cycles of time preceding even the appearance of man on the surface of our globe, that our own period seems as yesterday when compared with the epochs that have gone before it. Had we only the evidence of the deposits of rock heaped above each other in regular strata by the slow accumulation of materials, they alone would convince us of the long and slow maturing of God's work on the earth but when we add to these the successive populations of whose life this world has been the theatre, and whose remains are hidden in the rocks into which the mud or sand or soil of whatever kind on which they lived has hardened in the course of time,--or the enormous chains of mountains whose upheaval divided these periods of quiet accumulation by great convulsions,--or the changes of a different nature in the configuration of our globe, as the sinking of lands beneath the ocean, or the gradual rising of continents and islands above it,--or the wearing of great river-beds, or the filling of extensive water-basins, till marshes first and then dry land succeeded to inland seas,--or the slow growth of coral reefs, those wonderful sea-walls raised by the little ocean-architects whose own bodies furnish both the building-stones and the cement that binds them together, and who have worked so busily during the long centuries, that there are extensive countries, mountain-chains, islands, and long lines of coast consisting solely of their remains,--or the countless forests that must have grown up, flourished, died, and decayed, to fill the storehouses of coal that feed the fires of the human race to-day,--if we consider all these records of the past, the intellect fails to grasp a chronology for which our experience furnishes no data, and the time that lies behind us seems as much an eternity to our conception as the future that stretches indefinitely before us.