256 



ANCIENT GEOGRAPHICAL CHANGES. 



[Ch XII. 



and 



been formed at the termination of large hydrographical 

 basins, each, drained by a great river and its tributaries ; 

 the accumulation of sediment bears testimony to contempt 

 raneous denudation on a large scale, and, consequently, to 



r± -* "1 TIT I • • ill • • i 



area of land, probably containing within it 



a 



one or more 



mountain-chains . 



* 



In the case of the great Ohio or Appalachian coal-field, the 

 largest in the world, it seems clear that the uplands drained 

 by one or more great rivers were chiefly to the eastward, or 

 occupied a space now filled by part of the Atlantic Ocean, 



mecl 



mater 





eastern borders of the coal-field, or the south-east flanks of 



near Philadelphia — in other words, 



the Alleghany Mountains, near Philadelphia— 

 as we get nearer to the Atlantic. In that region numerous 

 beds of pebbles, often of the size of a hen's egg y are seen to 

 alternate with beds of pure coal. 



It has also been observed, in reference not only to the 

 Carboniferous but to the antecedent Devonian and Silurian 



of North 

 . from th 



limestones 



constantly in thickness, while the 



deposits, with corals and encrinites, increase and replace the 



others. 



American 



30th and 50th degrees of north latitude ; and there is no 



presume 



originated ever penetrated so far, or in such masses, into the 

 colder and arctic regions, as to generate a cold climate. 



m 



tain limestone, was of marine origin, and its occupancy 

 of large areas in Europe and the United States, and in parts 



of North A 

 conceivable 



may 



mi 



warmth and uniformity of climate throughout the globe. 



The Silurian strata now constituting parts of many upland 

 or mountainous regions in Europe and America were formed 

 for the most part in deep seas far from land, which may 











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