Fanning  of  Gloucestershire. 
123 
Feet. 
Limestone,  marl,  and  sandstone,  often  red  . . 400 
Central  portion,  chielly  thick-bedded  gray  limestone  . 1,438 
Lower  limestone  sliale  ; alternations  of  brown  shale, 
marl,  and  limestone  500 
2,338 
In  the  Forest  of  Dean  its  thickness  is  only  one-third  of  the  above  : — 
Feet. 
Upper  portion  ........  146 
Central  ........  480 
Lower  shales  .......  165 
791 
It  extends  below  the  whole  of  both  these  coal-districts,  and  its  outcrop 
forms  a zone  of  calcareous  hills  encircling  them,  with  the  exception  of 
certain  spaces  where  it  is  overlapped  by  the  coal-measures,  or  concealed 
by  the  new  red-sandstone. 
The  limestone  beds  consist  chiefly  of  corals,  encrinites,  and  shells,  im- 
bedded in  calcareous  matter,  itself  produced  from  the  destruction  of  skele- 
tons of  marine  animals.  Some  of  these  limestones  have  an  oolitic  struc- 
ture, others  are  dolomitic,  others  siliciferous,  and  there  are  beds  of  nodular 
chert  in  horizontal  layers  like  the  flints  in  the  upper  chalk. 
Near  Lower  Perlieu,  in  the  Forest  of  Dean,  the  limestone  contains  much 
hoematite  iron-ore,  which  has  long  been  extensively  worked ; but  neither 
the  lead-ore  nor  coal  so  abundant  in  the  same  stratum  as  in  the  north  of 
England. 
The  thick  compact  beds  are  a marble  much  used  for  mantel-pieces,  pil- 
lars, &c. ; it  is  extensively  quarried  near  Bristol  as  a road-stone,  for  which 
purpose  it  is  carried  as  far  as  Cirencester  and  Tewkesbury,  and  it  affords 
the  strongest  lime  in  the  county. 
Owing  to  its  great  hardness  the  soils  upon  this  rock  are  usually  thin,  and 
to  a great  extent  remain  in  sheep-walks ; the  natural  herbage  resembles 
that  of  the  more  elevated  parts  of  the  Cotswolds  and  Chilterns,  where 
sheep’s-feseue  and  burnet  abound  : but  the  carboniferous  limestone  has  a 
more  barren  and  subalpine  character,  from  the  absence  of  the  juniper, 
the  gorse,  and  the  fern,  which  often  clothe  the  oolite  and  chalk.  See 
Morton’s  account,  * Whitfield  Farm.’ 
5.  Millstone  Grit. — The  lowest  division  of  the  coal-measures  is  still  un- 
productive in  this  part  of  England,  and  hence  termed  by  the  miners  of 
South  Wales  the  “ Farewell  Rock.”  It  is  a coarse  quartzose  sandstone, 
much  used  for  millstones. 
In  the  Forest  of  Dean  it  is  from  265  to  455  feet  thick,  and  the  Bristol 
coal-field  from  950  to  1200  feet,  with  one  seam  of  coal  400  feet  from  the 
base.  The  soils  above  are  for  the  most  part  very  poor  and  sterile. 
C.  The  coal-measures  consist  of  alternating  beds  of  sandstone  and  argil- 
laceous shales,  with  occasional  marl-beds  and  coal-seams.  Beneath  every 
bed  of  coal  is  a peculiar  layer  of  sandy  clay,  from  a few  inches  to  4 or  5 
feet  in  thickness. 
The  Bristol  coal-field  (see  Map  and  Section)  may  be  divided  into  three 
portions — 
Feet. 
Upper  sandstone  and  shales  . 1,800 
Hard  sandstones,  called  “ Pennant  *’  . . . 1,725 
Lower  shales  . . . . . . . 1 , 5(55 
5,090 
