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AST summer it was the writer's privilege to lead a small expedition from Yale to the fossil fields of the west in search for the relics of bygone creatures to add to the already extensive collections owned by the university. Our purpose was not solely that of collecting, however, but to get data concerning the distribution in time of certain of the ancient faunas, hoping thereby not only to increase the sum of our knowledge, but to date more accurately some of the wealth of forms collected by the pioneer expeditions which, under the leadership of Professor Marsh, penetrated the unknown west in the early seventies.

The work was partly in Nebraska exploring the Tertiary rocks for the remains of warm-blooded mammals—horses, camels, rhinoceroses, elephants, and their kindred—and partly in eastern Wyoming, where one finds sediments of greater age containing the earthly inhabitants of the closing years of the Age of Reptiles. The mammal collecting is an old, old story, but the work in Wyoming had many novel features and forms the theme of this brief essay.

The Mesozoic rocks, those of the Age of Reptiles, are exposed with their contained fossils in many places in this broad earth of ours, but nowhere to a greater advantage than in the west, and this is particularly true of the states of Wyoming, Colorado and Montana. One of the counties of eastern Wyoming, formerly called Converse county, is now divided into two portions, of which the westernmost retains its ancient name, while the eastern part has been called Niobrara after the long Nebraska river whose source lies here. The latter county includes one of the most notable of Mesozoic localities, the beds lying on either side of the confluent Lance and Lightning creeks. The former of these is a tributary of the Cheyenne river, through which its waters flow into the Missouri on their long journey to the Gulf.

The strata here exposed belong to the ultimate phase of the Cretaceous period, marking the very close of the Reptilian age and possibly its passage into the Age of Mammals. They cover an area of more than sixty square miles, and, because of their geographical locality, have received the name of Lance formation, though they have been called variously Converse County beds and Ceratops beds, the latter name having reference to the most characteristic fossils, the horned dinosaurs or Ceratopsia, whose huge three-horned skulls are the most remarkable