152 



ON THE ORIGIN OF COAL-FIELDS. 



in lakes or the sea-shores, may form future coal-fields, upon any great changes taking 

 place between the present relative levels of land and water, by which the subaqueous 

 accumulations may be desiccated. 



The second is an ideal map representing the probable outline of a part of the region 

 under review, when the carboniferous series was accumulating. The Silurian and older 

 rocks are supposed to have been the dry land from which rivers flowed, occasionally 

 lodging the vegetable matter in partial lakes (a a) , before they discharged their contents 

 either into the estuaries (6 b), or into the broad sea marked " mare carboniferum " (c c), 

 under which the largest accumulations of coal previously described, were formed. The 

 area so designated, is that which is now covered by the New Red Sandstone of the central 

 counties, from beneath which the coal measures protrude at many points, as detailed in 

 this and other chapters. The reader will thus learn how some beds of coal may be 

 associated with freshwater, and some with sestuary productions, whilst others have been 

 formed amid accumulations exclusively of marine origin. 



