HOW IS COAL FORMED? 
55 
the roots of plants and vines. In the Kooky Moun¬ 
tains we often find brown lignite, which is wood par¬ 
tially changed to coal. These facts lead us to believe 
that the coal has been formed in swamps and low 
places, and in the presence of water. Indeed, when 
we come to examine the location of our Pennsylvania 
coal-fields, we learn that they occupy what were once 
swampy basins near the ocean, surrounded on three 
sides by high ground, but open on the southeast to¬ 
ward the sea. In these swamps and along their bor¬ 
ders flourished great ferns (Fig. 32) and trees in trop¬ 
ical abundance, for the climate of the age was warm 
and moist. As the vegetation decayed, it piled up in 
masses in the bottom of the marsh, and was preserved 
by the water and the acids produced by the decaying 
matter itself. On top of it sediments gradually formed, 
which helped to pack and preserve the vegetable 
matter beneath. After such a period the land, no 
doubt, subsided, allowing the ocean to overflow the 
marshy tracts. It must have remained under water 
for some time, as we now find layers of shale or lime¬ 
stone of varying thickness deposited on top of the 
coal-seams. Again the land rose, vegetation grew in 
the swamps and decayed and accumulated as before, 
until the land subsided once more. Thus the process 
was repeated as many times as there are seams of coal 
in that region. 
Geologists used to think that coal had been formed 
only at the mouths of rivers where vegetation had 
been washed down by freshets and piled up in great 
masses. This may have been true to some extent, but 
