Page:Popular Science Monthly Volume 22.djvu/146

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136
THE POPULAR SCIENCE MONTHLY.

at least are described by Dr. Dawson, consisting of conifers, ferns, sigillarids, cycads, lycopods, etc. Several interesting varieties of this Devonian flora have recently been discovered in Ohio. The fifth flora is the Carbonaceous, the sixth the Liassic, and the seventh the Cretaceous, a flora containing broad-leaved plants of the angiosperms, which, with slight change, has continued to the present time. The Jurassic group is plainly cretaceous, and is entirely unlike the Tertiary. Not a single plant can be identified with any of the Old World tertiary flora. The various tertiary floras of the Rocky Mountains occur in the dried-up basins of old lakes. When the glacial period came on, the trees were entirely destroyed over the northern part of the continent. After the melting of the ice, the present flora made its appearance.

The Depression of our Atlantic Coast.—Professor George H. Cook, State Geologist of New Jersey, has presented, in a paper which he has read before the American Association, a large array of evidence showing that the Atlantic coast of our continent is gradually subsiding. It consists largely of the testimony afforded by the remains of ancient forests, composed for a considerable part of upland growths, which have been found in various places from the Carolinas to Greenland, either submerged at high water or at depths beneath the surface lower than the high-water mark of the neighboring coast, and at these places sometimes with present or former swamps over them. Sunken forests possessing some or other of these characteristics are mentioned as existing in the Carolinas and Georgia, where they were noticed by Bartram in 1773, Lyell in 1845, Professor Tuorney, of South Carolina, in 1846, and in Albermarle Sound, North Carolina, by Dr. Emmons. General Cutts, of the Coast Survey, has observed timber in the place of its growth several feet below the level of tide-water along the shores of Chesapeake Bay, in Virginia. The coast of New Jersey is marked by the occurrence of timber and stumps below the present tide-level in the marshes which border the State from the head of Delaware Bay to Cape May, and thence to the mouth of the Hudson. A marked example of yellow-pine stumps may be observed in the banks of the canal which connects South River at Washington with the Raritan. Similar submerged forests on Long Island have been described by Elias Lewis, Jr., in "The Popular Science Monthly." In Massachusetts, they have been observed at Nantucket, Holmes's Hole, Yarmouth, and Provincetown; in New Hampshire, at Eye Beach; in Maine, at Portland; and at the head of the Bay of Fundy in Nova Scotia. Other evidences are afforded by the subsidence of human structures since the period of settlement; in the flooding of farm-lands that have had to be abandoned, the submersion of boat-stakes, and the approach of the sea to buildings on the shore. Instances of this kind are observable at Southampton, Long Island, Barnegat, New Jersey, the shores of Delaware Bay, and on the west coast of Greenland. The encroachments of the sea at Long Branch and the changes going on at Sandy Hook are public facts. Professor Cook believes that the change thus marked is common to the whole northern hemisphere. It is certainly taking place in parts of Sweden. Some doubt has been thrown upon the theory of a subsidence from the fact that sea-shells and buried timber, both of kinds now living, have been found in deposits a few feet above the present sea-level. These instances are, however, regarded as belonging to another era than the present period of depression, and are distinguished by several important differences of features from those now under consideration. Professor Cook thinks that they belong to a previous period of depression; that the present period may not have been going on for more than five hundred or a thousand years, and that in the one which preceded it the surface of the upland was ten feet or more nearer the sea-level than it now is. He adds: "A careful study of the numerous cases like this will satisfactorily prove that there have been other periods of alternate depression and elevation in comparatively recent times, the phenomena of which are so nearly alike that they are very commonly confounded with each other. And, when they are clearly distinguished, it will be found that the rise or the depression is one common to our whole coast, and probably to the whole northern hemisphere."