Throughout this epoch the mountains of North and South America were active, both the Andes and the southern ancestral Rocky Mountains rising. The great Atlantic and Pacific high coastal regions began to sink, eventually becoming so eroded and submerged that the coast lines of both oceans withdrew to approximately their present positions. The deposits of this inundation average about one thousand feet in thickness.
190,000,000 years ago witnessed a westward extension of the North American
Carboniferous sea over the present Rocky Mountain region, with an outlet
to the Pacific Ocean through northern California. Coal continued to be laid
down throughout the Americas and Europe, layer upon layer, as the coastlands
rose and fell during these ages of seashore oscillations.
180,000,000 years ago brought the close of the Carboniferous period, during
which coal had been formed all over the world—in Europe, India, China, North
Africa, and the Americas. At the close of the coal-formation period North America
east of the Mississippi valley rose, and most of this section has ever since
remained above the sea. This land-elevation period marks the beginning of the
modern mountains of North America, both in the Appalachian regions and in the
west. Volcanoes were active in Alaska and California and in the mountain-forming regions of Europe and Asia. Eastern America and western Europe were connected by the continent of Greenland.
Land elevation began to modify the marine climate of the preceding ages and to substitute therefor the beginnings of the less mild and more variable continental climate.
The plants of these times were spore bearing, and the wind was able to spread them far and wide. The trunks of the Carboniferous trees were commonly seven feet in diameter and often one hundred and twenty-five feet high. The modern ferns are truly relics of these bygone ages.
In general, these were the epochs of development for fresh-water organisms; little change occurred in the previous marine life. But the important characteristic of this period was the sudden appearance of the frogs and their many cousins. The life features of the coal age were ferns and frogs.
THE SEED-PLANT PERIOD
THE AGE OF BIOLOGIC TRIBULATION
This period marks the end of pivotal evolutionary development in marine life and the opening of the transition period leading to the subsequent ages of land animals.
This age was one of great life impoverishment. Thousands of marine species perished, and life was hardly yet established on land. This was a time of biologic tribulation, the age when life nearly vanished from the face of the earth and from the depths of the oceans. Toward the close of the long marine-life era there were more than one hundred thousand species of living things on earth. At the close of this period of transition less than five hundred had survived.
The peculiarities of this new period were not due so much to the cooling of the earth's crust or to the long absence of volcanic action as to an unusual combination of commonplace and pre-existing influences—restrictions of the
