Page:Popular Science Monthly Volume 73.djvu/561

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S early as 1878 it was known that the remains of a wonderful group of animals lay hid in the rocks of north central Texas, that had lived their appointed time and passed away before the earth history was half completed; since then collectors have gone into this region more or less regularly, contending in the early days with hostile Indians and later with bad water and difficult transportation. Since 1895 the author has made several trips, gathering vertebrate fossils for the University of Chicago and the American Museum of Natural History in New York. The descriptions below are based on these collections and those of earlier workers.

Perhaps one can get the best idea of the age of the rocks and the fossils by remembering that they were laid down in the portion of geological time called the Permian age, just after the period of the coal deposition. Reckoning the completed history of the earth as about one hundred millions of years, these rocks and fossils are from thirty to forty millions of years old. At the beginning of the Carboniferous age, when the coal was laid down, the part of the continent that is now called the Mississippi-Valley was covered by a wide sea, but during this age there was a progressive shallowing which culminated in the elevation of the Appalachian mountains in the east and the appearance of dry land from the new mountains on the east to the forebears of the Rockies on the west.

The appearance of dry land was at once the cause of the development of the wonderful group of Permian animals and the reason that so few are preserved to us, for it is only when the hard parts of animals or plants are buried in some water-soaked layer of the earth or are covered by water that they can be petrified and preserved. If they remain exposed to the air they are soon destroyed; so, of the skeletons of the thousands of buffalo left lying on the plains but a few years ago, there remain to-day but a few rotten and frost-split horns and bones. Undoubtedly in the muddy banks and bars of the rivers there are skeletons undergoing the slow process of petrifaction which will preserve them to be the chief treasure of some future museum. And so because of the land conditions which prevailed over so much of the