462 ELEMENTARY GEOLOGY 
It is not necessary to believe that these swamps were 
salt marshes, for they may have existed on lowland, 
partly elevated above sea-level, and covered by swamps, 
similar to the Dismal Swamp of Virginia and North 
Carolina, and the extensive swamps of Florida. 
Whatever the condition of the land on which the 
plants grew, they existed so near the level of the sea, 
and upon land so:unstable, that every now and then 
the area they occupied was lowered beneath the sea- 
level. Among the coal-bearing beds are found layers 
of coal covered with strata of sandstone, limestone, or 
clay, in which fossils of ocean animals abound. Hence 
we have recorded the alternation from swampy con- 
ditions to submergence by the ocean. ‘The land was 
therefore in a very unstable condition, now bemg above 
the sea, now beneath it. 
By slow transformation these buried beds of vegetable remains 
have been changed to coal. The volatile substances have dis- 
appeared, and the carbon and earthy ash have been left. This 
product is bituminous coal. In some-places the metamorphism 
has gone so far that the purer anthracite has been formed. This 
is especially developed where mountain folding has caused the 
changes. 
This instability of the shallow sea bottom and mar- 
ginal coast swamps, was accompanied and succeeded by 
the growth of the Appalachian Mountains. In fact, 
the unstable sea bottom was a sort of premonition 
