CHAPTER XIX 

 THE AGE OF REPTILES 



WE have already said that in the many hundreds 

 of thousands of years which went by during 

 Carboniferous times the sea sometimes ad- 

 vanced and sometimes receded, and nothing shows this 

 better than the great thickness of the deposits in which 

 the coal lies in seams. In America, as in Europe, Asia, 

 Africa, Australia, and New Zealand, South and Central 

 America, the Carboniferous system is found. In Arkansas, 

 in North America, the coal measures attain the re- 

 markable thickness of 18,000 feet ; in the Wasatch 

 Mountains the Carboniferous strata have been estimated 

 to be 13,000 feet thick, and in silver-bearing Nevada 

 10,000 feet. The formations of the Western European 

 coal measures, like those of Eastern North America, 

 consist principally of shales and clays, with smaller 

 amounts of sandstone and limestone. They attain great 

 thickness, and, including 5500 feet of the Millstone Grit, 

 are 13,500 feet thick in Lancashire and several thousand 

 feet thick in many parts of Great Britain and Ireland. 

 The extraordinary thicknesses show that near our islands 

 must have been a very extensive and lofty area of land. 

 In Germany the same strata, thickly seamed with coal, 



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