122 
Fanning  of  Gloucestershire. 
1.  The  Trap  rock  of  Tortworth  is  a volcanic  rock,  of  the  same  nature 
with  the  hard  varieties  of  modern  lava,  and  is  associated  with  the  oldest 
Silurian  rocks  of  the  county.  In  composition  it  is  very  variable,  passing 
from  compact  felspar  to  granitic  and  compact  greenstone,  claystone,  and 
amygdaloid  : its  colour  varies  from  black  to  green,  red,  brown,  and  gray. 
It  has  been  extensively  quarried  at  Woolford,  Horseley,  and  Micklewood 
Chace,  and,  when  not  already  in  a decomposed  state,  makes  the  most 
durable  road-stone  in  the  county.* 
2.  Silurian  rocks  occur  in  the  Vale  of  Berkeley,  extending  from  Tort- 
worth  to  Pyrton  Passage,  also  at  May  Hill,  Ledbury,  and  between  Much 
Marcle  and  Newent.  At  May  Hill  the  strata  have  the  following  order  and 
thickness : — 
Feet. 
Wenlock  limestone,  sandstone,  shale,  and  limestone  . 700 
Wenlock  shale  .......  1,000 
Woolhope  limestone  ......  240 
Caradoc  sandstone,  shale,  and  sandstone  . . .440 
2,380 
In  the  middle  of  the  Wenlock  limestone  series  is  a bed,  25  feet  thick,  of 
red  dolomitic  limestone  containing  28  per  cent,  of  carbonate  of  magnesia. 
See  Morton,  ‘ Silurian  Rocks  of  Whitfield.’ 
3.  Old  Red-Sandstone. — This  formation  occurs  near  Bristol  in  the  Vale 
of  Berkeley,  and  surrounds  the  Forest  of  Dean.  In  the  adjoining  counties, 
Hereford,  Brecon  and  Radnor,  &c.,  it  occupies  an  area  of  2100  square  miles, 
and  forms  hills — the  Beacon  of  Brecon,  2682  feet  high.  Between  Howl 
Hill,  in  Forest  of  Dean,  and  Much  Marcle,  its  thickness  has  been  mea- 
sured by  the  geological  surveyors : — 
Sandstone  and  conglomerate  . 
Sandstone  and  marls 
Marls  and  cornstone 
Feet. 
. 3,9*3 
790 
. 847 
5,620 
Cornstone  is  the  local  name  of  the  impure  limestone  which  is  sometimes 
found  in  this  formation. f 
The  marls  have  rendered  the  valleys  and  plains  of  Herefordshire  ex- 
tremely fertile  : they  occur  at  Whitfield  Farm.  See  Morton  on  Soils. 
But  the  conglomerate  whose  enormous  mass  surrounds  the  Forest  of 
Dean  forms  only  picturesque  hills  capable  of  very  little  cultivation.  The 
large  proportion  of  the  peroxide  of  iron,  which  causes  the  characteristic 
colour  of  the  formation,  does  not  appear  of  itself  to  be  prejudicial  to  vege- 
tation ; but  the  absence  of  almost  all  organic  remains  is  probably  attended 
by  a very  general  deficiency  of  carbonate,  and  still  more  of  phosphate,  of 
lime  in  the  soils  of  the  old  red-sandstone. 
4.  Carboniferous  Limestone. — Near  Bristol  this  formation  consists  of — 
* See  an  account,  by  Thomas  Weaver,  Esq.,  in  the  Trans.  Geo.  Soc. 
t Analysis  of  cornstone  and  red  marl : — 
Cornstone. 
Red  Marl. 
Carbonate  of  lime 
. . . . 69-3 
0-2 
Peroxide  of  iron 
. 2-2 
9-6 
Silica  . 
. . . . 19-5 
64*3 
Alumina 
. 7-2 
21-1 
Water 
. 09 
4-5 
'braces  of  chlorides, 
sulphates,  and  loss  . 0-9 
— 
Traces  of  chloride  of  sodium,  and  loss  . — 
0 3 
100 
100 
