times the British Isles were part of a desert extending over much
of northwestern Europe.

THE CRETACEOUS

The third great system of the Mesozoic includes many formations,
marine and continental, which record a long and complicated
history marked by great oscillations of the crust and wide changes
in the outlines of sea and land.

EARLY CRETACEOUS. In eastern North America the lowest Cretaceous
series comprises fresh-water formations which are traced from
Nantucket across Martha's Vineyard and Long Island, and through
New Jersey southward into Georgia. They rest unconformably on the
Triassic sandstones and the older rocks of the region. The
Atlantic shore line was still farther out than now in the northern
states. Again, as during the Triassic, a warping of the crust
formed a long trough parallel to the coast and to the Appalachian
ridges, but cut off from the sea; and here the continental
deposits of the early Cretaceous were laid.

Along the Gulf of Mexico the same series was deposited under like
conditions over the area known as the Mississippi embayment,
reaching from Georgia northwestward into Tennessee and thence
across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing
sea covered most of Mexico and Texas and extended a gulf northward
into Kansas. In its warm and quiet waters limestones accumulated
to a depth of from one thousand to five thousand feet in Texas,