Sometimes one finds the bone layer removed entirely,
sometimes horizontal, sometimes oblique, and again dipping directly
into the heart of the earth. This layer (dinosaur beds) is not more
than two hundred and seventy-four feet in thickness, and is altogether
of fresh-water origin; but as a proof of the oscillations of the
earth-level both before and after this great thin sheet of fresh-water
rock was so widely spread, there are evidences of the previous
invasion of the sea (ichthyosaur beds) and of the subsequent invasion
of the sea (mosasaur beds) in the whole Rocky Mountain region.
In traveling through the West, when once one has grasped the idea of
continental oscillation, or submergence and emergence of the land, of
the sequence of the marine and fresh-water deposits in laying down
these pages of earth-history, he will know exactly where to look for
this wonderful layer-bed of the giant dinosaurs; he will find that,
owing to the uplift of various mountain-ranges, it outcrops along the
entire eastern face of the Rockies, around the Black Hills, and in all
parts of the Laramie Plains; it yields dinosaur bones everywhere, but
by no means so profusely or so perfectly as in the two famous
localities we are describing.
_How the Skeletons Lie in the Bluffs and Quarry._ At the bluffs single
animals lie from twenty to one hundred feet apart; one rarely finds a
whole skeleton, such as that of Marsh's _Brontosaurus excelsus_, the
finest specimen ever secured here, which is now one of the treasures
of the Yale museum.
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