436 
MESSRS.  P.  LAKE  AND  T.  T.  GROOM  ON  THE 
[Aug.  1893, 
(a)  Bala  Beds. — In  the  area  under  consideration  these  consist 
entirely  of  bluish  slates.  They  are,  as  a  rule,  irregularly  cleaved  and 
of  little  economic  value  ;  but  two  quarries  have  been  opened  in  them 
close  to  Corwen,  and  one  at  least  of  these  is  still  worked.  In  some 
places  small  masses  of  soft  ferruginous  matter  occur  in  the  bedding- 
planes,  and  these  usually  weather  out  very  easily.  Near  Nant  Llechog 
this  ferruginous  matter  forms  regular  bands  a  few  inches  thick. 
Everywhere  these  slates  are  more  or  less  fossiliferous,  and  in 
many  places  fossils  are  extremely  abundant.  The  most  abundant 
in  the  upper  part  of  the  slates  are  Leptcena  sericea ,  Sow.,  Orthis 
elegantula ,  Dalm.,  Favosites  fibrosa ,  Goldf.,  and  other  Eavositid 
corals,  and  also  crinoids,  all  of  which  have  a  wide  vertical  range  in  the 
northern  part  of  the  Berwyns.  Orthis  calligramma ,  Dalm.,  and 
Pinnatopora  Bedgwicki ,  Shrubs.,  are  also  tolerably  common,  together 
with  Phyllopora  Hisingeri ,  M‘Coy,  and  Ptilodictga  dichotoma ,  Portl. 
In  addition  to  these  the  following  occur  more  or  less  frequently  : — 
Ptilodictga  explanata ,  M‘Coy,  Strophomena  rhomb oidalis,  AVilck., 
Btr.  simulans ,  M‘Coy,  Orthis  biforata ,  Schloth.,  0.  testuclinaria , 
Dalm.,  0.  protensa ,  Sow.,  0.  porcata ,  M‘Coy,  Orthoceras  ibex ,  Sow., 
Conularici  Sowerbyi ,  Defr.,  Illcenus  Bowmanni,  Salt.,  etc. 
Of  these  fossils,  some  are  wide-ranging  species  of  little  value  for 
determining  the  exact  horizon  of  the  beds  in  which  they  occur. 
Several  ( Orthis  biforata,  0.  protensa,  and  Phyllopora  Hisingeri )  are 
forms  which  belong  rather  to  the  Llandovery  and  Upper  Bala  Beds 
than  to  the  Middle  Bala  ;  while  others  ( Orthis  porcata ,  Ptilodictga 
explanata ,  and  Pinnatopora  Seclgwicki),  which  are  generally  re¬ 
cognized  as  Middle  Bala  forms  in  Britain,  occur  abundantly  in  the 
calcareous  grit  at  Glyn  Ceiriog  (Llandovery),  or  in  beds  imme¬ 
diately  below  it.  The  assemblage  of  fossils  has,  however,  a 
prevailing  Middle  Bala  aspect,  and  this,  together  with  the  fact  that 
the  characteristic  Upper  Bala  fossils  are  apparently  absent,  points 
to  the  conclusion  that  the  Upper  Bala  Beds  are  absent  at  Corwen, 
and  that  the  base  of  the  Llandovery  rests  here,  with  only  apparent 
conformity,  on  beds  fairly  high  up  in  the  Middle  Bala  Series. 
We  have  not  as  yet  distinguished  any  definite  zones.  A  little 
distance  below  the  grit  a  bed  occurs  which  is  full  of  Leptoena  sericea , 
while  almost  immediately  below  the  grit  Favosites  fibrosa,  Pinna¬ 
topora  Bedgwicki,  and  crinoids  appear  to  be  specially  abundant ;  but 
alJ  these  fossils  have  a  considerable  vertical  range.  The  Leptcma- 
bed  is  well  seen  at  the  eastern  end  of  the  hill  of  Bala  rocks  shown 
in  fig.  1  (p.  430),  and  is  also  found  at  the  side  of  the  stream  above 
(south  of)  the  Bectory. 
(b)  Corwen  Grit. — This  is  an  extremely  tough  and  compact  grit, 
quite  like  a  quartzite  in  texture.  It  is  almost  purely  siliceous, 
and  contains  none  of  the  felspathic  grains  which  are  so  abundant 
in  the  Denbighshire  Grits.  It  is  tolerably  uniform  in  character 
throughout  the  area,  and  owing  to  its  toughness  it  often  forms  a 
steep  scarp.  In  places,  for  example  just  west  of  Pen-y-pigin,  its 
structure  is  platy. 
We  have  not  succeeded  in  finding  any  fossils  in  this  bed,  and  if 
