^59:5.14 The land was periodically going up and down due to the shifting sea level occasioned by activities on the ocean bottoms. This crustal uneasiness — the settling and rising of the land — in connection with the prolific vegetation of the coastal swamps, contributed to the production of extensive coal deposits, which have caused this period to be known as the Carboniferous. And the climate was still mild the world over.

^59:5.15 The coal layers alternate with shale, stone, and conglomerate. These coal beds over central and eastern United States vary in thickness from 12 to 15 m. But many of these deposits were washed away during subsequent land elevations. In some parts of North America and Europe the coal-bearing strata are 5,500 m in thickness.

^59:5.16 The presence of roots of trees as they grew in the clay underlying the present coal beds demonstrates that coal was formed exactly where it is now found. Coal is the water-preserved and pressure-modified remains of the rank vegetation growing in the bogs and on the swamp shores of this faraway age. Coal layers often hold both gas and oil. Peat beds, the remains of past vegetable growth, would be converted into a type of coal if subjected to proper pressure and heat. Anthracite has been subjected to more pressure and heat than other coal.

^59:5.17 In North America the layers of coal in the various beds, which indicate the number of times the land fell and rose, vary from 10 in Illinois, 20 in Pennsylvania, 35 in Alabama, to 75 in Canada. Both fresh- and salt-water fossils are found in the coal beds.

^59:5.18 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 300 m in thickness.

^59:5.19 ¶ 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.

^59:5.20 ¶ 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.

^59:5.21 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.

^59:5.22 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 2 m in diameter and often 38 m high. The modern ferns are truly relics of these bygone ages.

^59:5.23 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.

6. THE CLIMATIC TRANSITION STAGE

THE SEED-PLANT PERIOD

THE AGE OF BIOLOGIC TRIBULATION

^59:6.1 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.

^59:6.2 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 100,000 species of living things on earth. At the close of this period of transition less than 500 had survived.

^59:6.3 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 seas and increasing elevation of enormous land masses. The mild marine climate of former times was disappearing, and the harsher continental type of weather was fast developing.

^59:6.4 ¶ 170,000,000 years ago great evolutionary changes and adjustments were taking place over the entire face of the earth. Land was rising all over the world as the ocean beds were sinking. Isolated mountain ridges appeared. The eastern part of North America was high above the sea; the west was slowly rising. The continents were covered by great and small salt lakes and numerous inland seas which were connected with the oceans by narrow straits. The strata of this transition period vary in thickness from 300 to 2,130 m.

^59:6.5 The earth’s crust folded extensively during these land elevations. This was a time of continental emergence except for the disappearance of certain land bridges, including the continents which had so long connected South America with Africa and North America with Europe.

^59:6.6 Gradually the inland lakes and seas were drying up all over the world. Isolated mountain and regional glaciers began to appear, especially over the Southern Hemisphere, and in many regions the glacial deposit of these local ice formations may be found even among some of the upper and later coal deposits. Two new climatic factors appeared — glaciation and aridity. Many of the earth’s higher regions had become arid and barren.

^59:6.7 ¶ Throughout these times of climatic change, great variations also occurred in the land plants. The seed plants first appeared, and they afforded a better food supply for the subsequently increased land-animal life. The insects underwent a radical change. The resting stages evolved to meet the demands of suspended animation during winter and drought.

^59:6.8 ¶ Among the land animals the frogs reached their climax in the preceding age and rapidly declined, but they survived because they could long live even in the drying-up pools and ponds of these far-distant and extremely trying times. During this declining frog age, in Africa, the first step in the evolution of the frog into the reptile occurred. And since the land masses were still connected, this prereptilian creature, an air breather, spread over all the world. By this time the atmosphere had been so changed that it served admirably to support animal respiration. It was soon after the arrival of these prereptilian frogs that North America was temporarily isolated, cut off from Europe, Asia, and South America.

^59:6.9 The gradual cooling of the ocean waters contributed much to the destruction of oceanic life. The marine animals of those ages took temporary refuge in three favourable retreats: the present Gulf of Mexico region, the Ganges Bay of India, and the Sicilian Bay of the Mediterranean basin. And it was from these three regions that the new marine species, born to adversity, later went forth to replenish the seas.