Were these sets of rocks found always in the regular sequence in which I have enumerated them, their relative age would be easily determined, for their superposition would tell the whole story: the lowest would, of course, be the oldest, and we might follow without difficulty the ascending series, till we reached the youngest and uppermost deposits. But their succession has been broken up by frequent and violent alterations in the configuration of the globe. Land and water have changed their level,--islands have been transformed to continents,--sea-bottoms have become dry land, and dry land has sunk to form sea-bottoms,--Alps and Himalayas, Pyrenees and Apennines, Alleghanies and Rocky Mountains, have had their stormy birthdays since many of these beds have been piled one above another, and there are but few spots on the earth's surface where any number of them may be found in their original order and natural position. When we remember that Europe, which lies before us on the map as a continent, was once an archipelago of islands,--that, where the Pyrenees raise their rocky barrier between France and Spain, the waters of the Mediterranean and Atlantic met,--that, where the British Channel flows, dry land united England and France, and Nature in those days made one country of the lands parted since by enmities deeper than the waters that run between,--when we remember, in short, all the fearful convulsions that have torn asunder the surface of the earth, as if her rocky record had indeed been written on paper, we shall find a new evidence of the intellectual unity which holds together the whole physical history of the globe in the fact that through all the storms of time the investigator is able to trace one unbroken thread of thought from the beginning to the present hour. [Illustration: SKELETON OF ANIMAL OF THE MIOCENE PERIOD.] [Illustration: SKELETON OF ANIMAL OF THE PLIOCENE PERIOD.]