vene.

From this diagram it will appear that the six cycles or periods were
by no means equal in the amount of overflow or complete recovery of
the drowned lands. The Cretacic period was marked by a much more
extensive and long continued flooding; the great plains west of the
Mississippi were mostly under water from the Gulf of Mexico to the
Arctic Ocean. The earlier overflows were neither so extensive nor so
long continued. The great uplift of the close of the Cretacic regained
permanently the great central region and united East and West, and the
overflows of the Age of Mammals were mostly limited to the South
Atlantic and Gulf coasts.

_Sedimentary Formations._ During the epochs of greatest overflow great
marine formations were deposited over large areas of what is now dry
land. These were followed as the land rose to sea level by extensive
marsh and delta formations, and these in turn by scattered and
fragmentary dry land deposits spread by rivers over their flood
plains. In the marine formations are found the fossil remains of the
sea-animals of the period; in the coast and delta formations are the
remains of those which inhabited the marshes and forests of the coast
regions; while the animals of the dryland, of plains and upland, left
their remains in the river-plain formations.

[Illustration: Fig. 5.--Geologic Cycles and the Land Area of North
America (after Schuchert).]

These last, however, fragmentary and loose and overlying the rest,
were the first to be swept away by erosion during the periods of
elevation; and of such formations in the Age of Reptiles very little,
if anything, seems to have been preserved to our day. Consequently we
know very little about the upland animals of those times, if as seems
very probable, they were more or less different from the animals of
the coast-forests and swamps. The river-plain deposits of the Age of
Mammals on the other hand, are still quite extensive, especially those
of its later epochs, and afford a fairly complete record in some parts
of the continent of the upland fauna of those regions.

_Occurrence of Dinosaur Bones._ Dinosaur bones are found mostly in the
great delta formations, and since those were accumulated chiefly in
the early stages of great continental elevations, it follows that our
acquaintance with Dinosaurs is mostly limited to those living at
certain epochs during the Age of Reptiles. In point of fact so far as
explorations have yet gone in