440  LLANDOVERY  AND  ASSOCIATED  ROCKS  OF  CORWEN.  [Aug.  1893, 
geological  mapping,  and  it  was  fortunate  for  the  Authors  that  such 
a  rock  occurred  in  the  area  they  had  described. 
Prof.  Hughes  said  that  one  of  the  great  difficulties  in  tracing  this 
basement-series  was  that  it  rapidly  varied  horizontally  ;  that  fossils 
were  found  only  here  and  there  in  lenticular  beds  and  patches  ;  and 
that  the  border-ground  was  much  disturbed.  Therefore  a  careful 
bit  of  mapping  like  that  offered  by  Messrs.  Lake  and  Groom  was 
of  the  greatest  value.  He  thought  that  the  Hirnant  Limestone 
should  be  divided  into  (1)  an  upper,  sandy,  pyritous,  calcareous, 
often  pisolitic  rock,  weathering  into  a  gingerbread-coloured  mass, 
which  was  the  equivalent  of  the  Corwen  Grit  and  of  the  calcareous 
grit  of  Ponthafodcynfor ;  (2)  a  lower  portion  consisting  of  bands 
of  limestone  or  limestone-nodules  in  cleaved  rock  which  belonged 
to  the  Bala  Beds  ;  and  that,  whether  by  the  discordant  creep¬ 
ing  of  the  basement-bed  of  the  Silurian  over  the  various  members 
of  the  Upper  Bala,  or  by  the  dying  out  of  the  nodular  limestone,  it 
was  only  here  and  there  seen,  as  at  Aberhirnant,  immediately  under¬ 
lying  the  basement-bed  to  which  the  name  ‘  Hirnant  Limestone  ’ 
should  properly  be  confined. 
He  enumerated  some  of  the  characteristic  fossils  of  that  horizon, 
pointing  out  some  sources  of  error,  if  surface-specimens  were  col¬ 
lected,  arising  out  of  the  circumstance  that  casts  only  were  pre¬ 
served  and  those  mostly  fragmentary,  while  there  were  unfortunately 
some  more  or  less  close  resemblances  between  certain  fossils  of  the 
adjoining  horizons, — such  as  Orthis  Tiirnantensis  and  tStrophomeiyx 
siluriana ;  Ortliis  hiforata  and  0.  spiriferoides ;  the  dwarfed  and 
clean-cut  variety  of  Meristella  crctssa  and  M.  tumida ,  M.  angusti- 
frons,  and  others.  He  thought  that  Pentamerus  lyrcitus  occurred 
in  North  Wales,  but  he  had  never  been  able  to  verify  the  occurrence 
of  P.  oblongus  in  the  northern  area. 
Mr.  Groom  agreed  with  Prof.  Hughes  as  to  the  difficulty7  of 
obtaining  palaeontological  evidence  to  decide  the  age  of  the  grit  at 
Corwen ;  and  said,  with  reference  to  the  occurrence  in  the  northern 
part  of  the  Berwyn  Hills  of  the  unconformity  mentioned  by  Prof.  Lap- 
worth  as  existing  in  the  south-east,  that  the  Authors'  researches 
were  still  insufficient  to  show  whether  Upper  Bala  rocks  were  absent 
or  not  from  the  whole  region,  although  the  evidence  at  Corwen  itself 
seemed  distinctly  in  favour  of  a  break. 
Prof.  Lapworth  also  spoke,  and  Mr.  Lake  briefly  replied. 
