THE AMPHIBIA 8/ 



regularly below the sea to be overlaid with rich alluvial 

 soil to a depth of several thousand feet, forming three 

 great deltas, one in east-central Pennsylvania, another 

 in West Virginia, and a third in northern Alabama. As 

 these alluvial deposits grew in depth, the shallow sea 

 became broken into bayous, and ultimately the salt 

 water was displaced by fresh-water streams and inter- 

 lacing lakes, to give rise to the vast swamps which were 

 to be transformed into coal. Because it was in this period 

 that the great coal deposits were formed all over the 

 world, and because it is in Pennsylvania that the greatest 

 coal veins in the Western Hemisphere are found, the 

 period is designated the Pennsylvanian by American 

 geologists. 



The Mississippian climate of Europe and North 

 America was for the most part a semiarid one, relieved 

 only by short if intense seasonal rains. Large areas of 

 both hemispheres were covered with landlocked salt 

 lakes in which deep beds of salt were deposited, or by 

 deserts that spread from the subtropical regions into the 

 present Temperate Zones. On the whole it was not a 

 climate conducive to the multipKcation and diversifica- 

 tion of the Devonian terrestrial forms, though the tetra- 

 pods left traces of their existence in the form of 

 footprints in the Upper Mississippian red beds of Penn- 

 sylvania—associated with mud cracks and marks of the 

 raindrops presaging the short-lived flood that later 

 buried them— perhaps a record of a tragic search for 

 water as the mudholes gave way to hard-baked flats in 

 the summer sun. 



The Pennsylvanian, however, saw a remarkable trans- 

 formation in climate. The annual mean temperature of 

 Europe and North America is estimated to have risen to 

 53° F., or 20° higher than it is today, partly because the 

 spread of oceanic currents carried warm tropical water 

 so far into the Arctic region that marine corals grew as 

 far north as the now glacier-capped island of Spits- 



