Now  how  were  the  Carboniferous  rocks  formed  ?
They  consist  of  strata  partly  of  fresh-water  but  chiefly
of  marine  origin,  for  not  only  are  the  limestones  formed
chiefly  of  corals  and  shells,  but  many  of  the  shales
also  yield  similar  fossils.  Beds  of  coal  are  numerous,
(whence  the  name  Coal-measures,  originally  derived


Carboniferous  Rocks.  45

from  the  miners)  and  under  each  bed  of  coal  there  is
a  peculiar  stratum,  which  often,  but  not  always,  is  of
the  nature  of  fire-clay.  Sometimes,  it  is  called  '  under-
clay,'  this  being  the  miners'  term,  on  account  of  its
position  beneath  each  bed  of  coal.  Coal  itself  is  well
known  to  consist  of  mineralised  vegetable  matter,  and
when  you  examine  the  shales  and  sandstones  associated
with  it,  you  frequently  find  in  them  quantities  of
vegetable  remains,  ferns,  stems  of  reed-like  plants
(Catamites),  trunks  of  various  trees,  &c.  When  the
fire-clay  is  narrowly  examined,  you  also  generally  find
in  it  a  number  of  portions  of  plants  called  Stigmaria,
now  known  to  be  the  roots  of  a  fossil  tree  called  Sigil-
laria,  and  this  led  Sir  William  Logan,  Mr.  Binney,  and
other  geologists  to  infer  that  the  under-clay  was  the
original  soil  on  which  the  plants  grew,  the  decay  and
subsequent  mineralisation  of  which  formed  beds  of  coal.
Among  those  plants  which  are  found  in  the  coal  and
its  associated  under-clay,  and  in  the  shale,  may  be  enu-
merated the  following  genera:  Sigillaria,  Lepidodendron,
Ulodendron,  Calamites,  Halonia,  &c.,  and  numerous
genera  of  Ferns.
Now  in  the  Scottish  Coal-measures  there  are  in
Edinburghshire1  over  3,000  feet  of  coal-bearing  strata,
so  that  the  lowest  bed  of  coal  may  be  nearly  three
thousand  feet  below  the  highest  bed,  in  the  centre  of
the  basin,  where  the  strata  are  thickest.  Most  of  them


46  Carboniferous  Rocks.
