Page:Popular Science Monthly Volume 34.djvu/32

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land ranges, crept southward to Long Island and northern New Jersey, and heaped up the eastern extension of the terminal moraine, and, as it melted, gave origin to the Champlain deposits of the New England rivers and to certain distinctive aqueo-glacial gravels, of which those of the Delaware at Trenton are the type.

The lower lacustral deposits of the Great Basin, the aqueo-glacial deposits of the Mississippi Valley, and the ancient deltas of the Atlantic slope are correlated, partly because (1) each attests a great and similar climatal episode, because (2) it is evident that each of these episodes was so extreme as to affect the entire breadth of the continent, and because (3) there are no indications among American geologic deposits of other episodes with which these might be confused; the upper lacustral beds of the Great Basin, the upper glacial and aqueo-glacial deposits of the Mississippi Valley, and the glacial deposits of the Atlantic slope, are correlated upon similar grounds; and the harmony among the various records gives cumulative proof of the accuracy of each.

By these researches of the last decade the earlier conceptions of Quaternary history are greatly expanded, and the hitherto obscure relation between the Tertiary and Quaternary is made clear. Where they contain vertebrate fossils, the earlier and even the later of these deposits are, it is true, referred to the Pliocene by paleontologists, while physical geologists refer the entire series to the Quaternary; but this discrepance is one of classification only, and in no way affects the phenomena classified.

The sequence of events made out independently in the three widely separated regions may be depicted as in the accompanying diagram (Fig. 1), representing the temperatures and land-altitudes

PSM V34 D032 Graph of climate and land altitude changes.png

Fig. 1.—Graphic Representation of Climate and Changes in Altitude of the Land.

during late Tertiary and Quaternary time. By such a chronograph alone is it possible to accurately measure the antiquity of paleolithic man. Marsh has well shown that only long æons can be measured by plant fossils, that somewhat shorter periods may be measured by the records of invertebrate life, and that the vertebrates afford by far the most delicate of the paleontologic time-measures; but the swing of even the vertebrate life-pendulum is