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Page:Popular Science Monthly Volume 20.djvu/254

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hundred to five hundred feet, and then unconformably overlaid by the orange sand-a deposit coeval with the glacial drift. Borings in lower Louisiana show first the Port Hudson group or post-glacial deposits; second, the orange sand; and, third, the underlying eroded Tertiaries with their characteristic fossils. This excavation could not have been made unless the waters of the Gulf had been at least five hundred feet lower than at present.

Other facts further illustrate these former elevations. Upon the Atlantic border between New Orleans and New York we find a low, broad plain, largely consisting of marshy or drowned lands. On reaching Virginia this land begins to disappear, but the plain holds the same relation to the continent, as it continues to exist in the submarine banks east of Massachusetts. Curiously enough, we now dredge from the George's and Great Banks Tertiary fossils similar to those occurring in the unsubmerged parts of this plain. Their existence beneath the ocean had never been suspected till the possible identity of the Atlantic plain with the eastern shoals had been suggested, subject to the crucial test of dredging.

These facts authorize us to believe that the eastern half of the continent has been elevated certainly six hundred feet in the glacial period. As this elevation included Northern Europe, and consequently the polar districts, so as to unite the Old and the New Worlds, we may be able to follow the older writers, and find in this land-mass the conditions adequate to produce the glacial cold. Such a cause will explain the facts more satisfactorily than the invocation of the eccentricity of the earth's orbit, or the precession of the equinoxes. If we can combine the two classes of causes, we can certainly explain all the phenomena, besides obtaining the data for the chronology of the ice age.

The Melting of the Glacial Sheet.—Time fails us to describe the marvelous facts connected with the melting of the ice. The numerous kames, elevated sand-plains, and river-terraces, found every-where in Northern America, came into existence with the copious floods of water resulting from the dissolution of the ice. The history of the ice age is incomplete without a discussion of the events occurring in this great continental freshet; but this sketch must be deferred for the present.



FOR the past two years I have had charge of a public school in Pennsylvania, and have endeavored to awaken in the minds of my pupils a love for and an interest in science, with especial reference to the truths and lessons of physiology and zoölogy. Perhaps my