524 STEVENSON— THE FORMATION OF COAL BEDS. [N°v. i. 



elusion is that the depth of water was never more than lOO feet in 

 those localities and generally much less. His studies led him to 

 conclude that the average depth of Palaeozoic seas was even less 

 than 200 feet and that none attained a depth exceeding 600 feet. 



There is no reason to suppose that the non-marine limestones are 

 other than shallow water deposits ; they are sun-cracked and rippled 

 in some extensive areas. Equally there appears to be no valid 

 reason for supposing tha:t the marine limestones are of deep sea 

 origin ; at times, they are sun-cracked and wave-marked ; at others 

 they are distinctly near-shore deposits ; yet the fauna, characier- 

 izing them at a distance from the shore, is present and the in- 

 dividuals are of such size as to show that the conditions were not 

 unfavorable. The only ground for asserting that the limestones are 

 of deep sea origin is the time-honored conception that presence of 

 certain groups of invertebrates is proof that the water was deep. 

 This conception itself stands seriously in need of proof. 



Mode of Deposition. 



Before taking up the study of coal beds, it is necessary to ascer- 

 tain, if possible, the conditions under which the deposits already 

 considered were laid down. Three possible hypotheses have been 

 suggested. 



The basin may have been a Alediterranean sea, 250 miles wide 

 and more than 800 miles long. 4,000 to 6,000 feet deep, into which 

 streams delivered debris until the whole area was filled. 



The basin, originally almost wholly dry land, was brought under 

 water by gradual but interrupted subsidence; inflowing streams 

 formed deltas which eventually filled the basin. 



The water-area, during most of the period, may have been com- 

 paratively insignificant. The two longitudinal valleys may have 

 had each its own important river, with tributaries, which formed 

 dejection cones, to be remodeled by floods and by meanders of the 

 streams, while the whole region was subsiding slowly though not 

 continuously. 



The first hypothesis is altogether improbable. It involves the 

 conception that the surface of the sea within the basin was at tide 



