IV. Tertiary Group of | Strata, 3,000 feet. | Eocene, Miocene, Pliocene. ---------------------------------+------------------------------ | IX. Chalk System. III. Mesolithic Group of Strata. | | Deposits of the | VIII. Jura System. Secondary Epoch, about | 15,000 feet. | | VII. Trias System. ---------------------------------+------------------------------ | | VI. Permian System. II. Palæolithic Group of Strata. | | Deposits of the | V. Coal System. Primary Epoch, about | 42,000 feet. | IV. Devonian System. ---------------------------------+------------------------------ | | III. Silurian System, about | 22,000 feet. I. Archilithic Group of Strata. | | Deposits of the | II. Cambrian System, about Primordial Epoch, about | 18,000 feet. 70,000 feet. | | I. Laurentian System, about | 30,000 feet. The striking differences which so frequently occur between the petrifactions of two strata, lying one above another, are to be explained in a simple and easy manner by the supposition that the same part of the earth’s surface has been exposed to _repeated depressions and elevations_. Such alternating elevations and depressions take place even now extensively, and are ascribed to the heaving of the fiery fluid nucleus against the rigid crust. Thus, for example, the coast of Sweden and a portion of the west coast of South America are constantly though slowly rising, while the coast of Holland and a portion of the east coast of South America are gradually sinking. The rising as well as the sinking takes place very slowly, and in the course of a century sometimes only amounts to some few lines, sometimes to a few inches, or at most a few feet. But if this action continues uninterruptedly throughout hundreds of thousands of years it is capable of forming the highest mountains.