78 EVOLUTION OF THE EARTH 



square miles. The greatest floods have spread from the Arctic 

 Ocean, others from the Pacific, and those of least amount from 

 the Atlantic. On the other hand, the number of floods was 

 greater from the south than from the north, and the waters 

 were most persistent in the Appalachian, lower Mississippi, 

 and Rocky Mountain areas. Where the greatest thicknesses 

 of sediments accumulate, out of this continental debris there 

 will arise, phoenix-like, a future mountain range. After about 

 40,000 feet of strata had been deposited in the Appalachian 

 trough, there arose near the close of the Paleozoic era the 

 majestic Appalachian Mountains, some of which may have 

 towered to at least 20,000 feet above the sea. In fact, the 

 original mountains were destroyed by erosion during the Meso- 

 zoic era and the reelevated Appalachians of today are due to 

 later uplifts of more than 2,000 feet; subsequent erosion has 

 developed them into their present interesting forms. Today 

 one may travel comfortably over the roots of these once grand 

 mountains by way of the Pennsylvania, the Baltimore and 

 Ohio, and the Chesapeake and Ohio railways. 



Areal 'variability of organic habitats. We have seen that 

 the dry lands become alternately larger and smaller, and in the 

 same way the shallow-water areas of the oceans are vastly 

 increased when they spread over the lands, and are again 

 greatly decreased when the land areas increase in size. Thus 

 when the lands are widely flooded there is a great increase in 

 the quantity of marine life, but little is originated that is new, 

 while on the lands the climate becomes insular, warm and 

 moist, and productive of the greatest amount of life in the 

 then restricted areas. On the other hand, when crustal read- 

 justments occur, the lands are largest, driest, and coolest, and 

 these changing environments react on the life and bring about 

 great alterations in the composition of the floras and faunas. 

 At these times the shallow-water marine areas are smallest 

 and most variable in turbidity and salinity conditions that 



