and of more than ten thousand feet in Mexico. Meanwhile the
lowlands, where the Great Plains are now, received continental
deposits; coal swamps stretched from western Montana into British
Columbia.

THE MIDDLE CRETACEOUS. This was a land epoch. The early Cretaceous
sea retired from Texas and Mexico, for its sediments are overlain
unconformably by formations of the Upper Cretaceous. So long was
the time gap between the two series that no species found in the
one occurs in the other.

THE UPPER CRETACEOUS. There now began one of the most remarkable
events in all geological history,--the great Cretaceous
subsidence. Its earlier warpings were recorded in continental
deposits,--wide sheets of sandstone, shale, and some coal,--which
were spread from Texas to British Columbia. These continental
deposits are overlain by a succession of marine formations whose
vast area is shown on the map, Figure 260. We may infer that as
the depression of the continent continued the sea came in far and
wide over the coast lands and the plains worn low during the
previous epochs. Upper Cretaceous formations show that south of
New England the waters of the Atlantic somewhat overlapped the
crystalline rocks of the Piedmont Belt and spread their waste over
the submerged coastal plain. The Gulf of Mexico again covered the
Mississippi embayment, reaching as far north as southern Illinois,
and extended over Texas.

A mediterranean sea now stretched from the Gulf to the arctic
regions and from central Iowa to the eastern shore of the Great
Basin land at about the longitude of Salt Lake City, the Colorado