191 1.] STEVENSON— FORMATION OF COAL BEDS. 29 



with mighty rivers, along whose coasts and in basin shaped depres- 

 sions was deposited the vast system of sand and mud strata: This at 

 length became marshland, ofifering the ground for the luxuriant 

 vegetation of the first coal bed. In the Appalachian region, there 

 may have been the flat coast of a land extending far to the east, 

 from which great rivers carried sand and mud into the shallow sea 

 at the west, in which, farther away, limestone was forming. Proc- 

 esses such as those now seen in the Nile, Mississippi, Hoangho 

 and other rivers, continuing for many thousands of years, would 

 raise the sea bed until it reached the water surface as a wide-spread 

 marshland. Similar operations were going on in freshwater basins 

 of the dry land leading to the formation of morasses, supporting 

 Calamites, SigUIaricc and other Carboniferous plants, which would 

 give a deposit of peat. 



The alternation of a great number of coal beds with thick masses 

 of sandstone and shale is not so easily explained as is the origin 

 of the first coal bed. The causes in paralisch areas are difi^erent 

 from those in limnisch basins. 



Lyell, Lindley and others held the opinion that seacoasts, on 

 which paralisch deposits were formed, underwent slow subsidence 

 during Carboniferous time. If one suppose that this subsidence was 

 interrupted periodically, we have a mechanism by which the forma- 

 tion of successive coal beds could be explained. A similar result 

 would be secured by occasional elevations of the sea-bottom, ac- 

 cording to Petzholdt's conception. There is necessary in each case 

 a general rise of the sea-level to cover the plant deposit with the 

 sandstone and shale needed to give another swampy surface. This 

 alternating subsidence and stability of the sea-bottom explains why 

 the shale, covering coal beds, encloses a mass of plant remains 

 and also why paralisch territories may have many but thin coal 

 beds. 



This explanation is not wholly satisfactory for limnisch areas, 

 since one can hardly suppose that all of those could have suffered 

 the repeated subsidence. One must conceive that between longer 

 periods of stability there were epochs in which increased fall of 

 inflowing streams or a diversion of flow occurred. The greater 

 carrying power of the streams would bring the plant deposit and 



29 



