I9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 523 



meters thick, is formed of very fine sediments. Clay shales make 

 up about 70 per cent, of the column ; nevertheless the Upper Coal 

 Measures of the Franco-Belgian basin cannot be regarded as a 

 deposit of the deep sea. He feels compelled to believe that the 

 neighboring areas were lowlands and that the continent was in an 

 advanced stage of peneplanation. 



Jukes-Brown had reached the same conclusion respecting the 

 English coal terrain. 



Schuchert/^^ in the work already cited frequently, asserts that, 

 during the periodic invasions in the Palaeozoic, the depth of water 

 in the Appalachian sea was very shallow, rarely exceeding 200 to 

 300 feet. On the west shore of Appalachia,. conglomerates, sand- 

 stone and coarse muds, with rippled surfaces, are common; while 

 in calcareous periods, one finds shrinkage cracks, marking great mud 

 flats inundated periodically with calcareous materials nearly devoid 

 of life. In the New York basin, the northern division of the Ap- 

 palachian, the later deposits are sands and muds without marine life, 

 though containing some land plants, some fishes and some fresh- 

 water bivalves. The sands are often red, oxidized materials in 

 estuaries, dried out by sun and air. The shallowness of the sea is 

 evidenced by the almost endless list of formation names applied by 

 field geologists. These conditions existed at the close of the 

 Devonian. 



Ulrich^^* in some instances would go farther than Schuchert in 

 limitation of depth. These students had covered a very great part 

 of the United States either by personal observation or by study of 

 collections made by government geologists and others. They agree 

 wholly in asserting that seas caused by ocean invasions were 

 shallow, but Ulrich feels justified in admitting for some extensive 

 areas a less depth than that which Schuchert, with abundant caution, 

 had named as a probable maximum in his general statement. He 

 had examined about 20 marine embayments of Ordovician and 

 Silurian age in the Nashville and Ozark uplifts — the former within 

 the area of Cincinnatia and the latter much farther west. His con- 



"^ Schuchert. " Palaeography of North America." pp. 438, 439. 

 "* E. O. Ulrich, " Revision of the Palaeozoic Systems," pp. 361. 



