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MESSES.  P.  LAKE  AKD  T.  T.  GEOOM  OK  THE 
[Aug.  1893, 
34.  The  Llandoyeey  and  Associated  Rocks  of  the  Heighbottehood 
of  Coewex.  By  Philip  Lake,  Esq.,  M.A.,  E.G.S.,  and  Theo. 
T.  Geoom,  Esq.,  B.Sc.,  E.G.S,  (Read  May  10th,  1893.) 
Contents. 
I.  Introduction . . 
II.  General  Sequence  of  the  Strata . 
III.  Structure  of  the  Area  . 
IV.  Description  of  the  Strata  . 
V.  List  of  Fossils  found  at  and  near  Corwen 
I.  Inteodectioh. 
It  was  for  many  years  a  doubtful  point  whether  true  Llandovery 
rocks  were  represented  in  the  more  northern  part  of  Wales,  or 
whether  the  Tarannon  Shales  here  rested  directly  on  the  Bala  Beds. 
Thus,  according  to  the  first  edition  (1866)  of  vol.  iii.  of  the  Geolo¬ 
gical  Survey  Memoirs  (p.  205  et  seq.),  no  Llandovery  rocks  are 
known  between  Conway  and  the  country  east  of  Bala  Lake;  and 
the  Lower  Llandovery  first  appears  near  Cefn-bwlan.  But  it  is  at 
the  same  time  noted  in  the  appendix  (p.  275)  that  Lower  Llan¬ 
dovery  fossils  occur  on  Cyrn-y-brain  (north  of  Llangollen)  and, 
doubtfully,  near  Pwllheli. 
In  a  paper  published  in  1877,  however,  Prof.  T.  MCK.  Hughes1 
described  a  series  of  grits  at  Corwen  which  rest  upon  Bala  Beds, 
and  are  overlain  by  pale  beds  passing  up  into  the  ‘  Pale  Slates  ’ 
(Tarannon  Shales)  of  the  Survey.  To  these  grits  he  gave  the  name 
of  Corwen  Grits.  Hear  Corwen  itself  he  found  in  them  no  fossil 
except  Favosites  alveolaris  (doubtful) ;  but  a  similar  grit  occurs  in 
the  same  stratigraphical  position  in  various  other  localities,  and  on 
Cyrn-y-brain  he  obtained,  in  fragments  of  what  seems  to  be  the 
same  grit,  Petraia  subdujolicata ,  P.  crenulata ,  and  MeristeUa  crassa. 
Hence  he  concludes  that  the  Corwen  Grit  is  of  Llandovery  age,  and 
that  it  forms  the  base  of  the  Silurian  in  this  area. 
Two  years  later  Mr.  T.  Ruddy2  described  a  number  of  sections  in 
the  same  district.  Everywhere  he  finds  that  the  Tarannon  Shales  are 
underlain  by  soft  blue  slaty  shales,  below  which  is  a  series  of  beds 
somewhat  variable  in  character,  but  always  more  or  less  gritty  or  cal¬ 
careous.  This  gritty  series  frequently  contains  numerous  fossils 
similar  to  those  of  the  Hirnant  Limestone,  such  as  Orthis  biforata , 
0 .  hirnantensis ,  0.  sagittifera ,  Nebuliporct  lens ,  etc.  He  shows 
that  the  Lower  Llandovery  Grits  of  Cefn-bwlan  occupy  the  same 
position  with  respect  to  the  Tarannon  Shales ;  and  it  is  clear,  from 
Prof.  Hughes’s  observations,  that  the  Corwen  Grit  does  the  same. 
In  his  paper  on  the  Dee  Valley,  Mr.  J.  E.  Marr3  shows  that  the 
Corwen  Grit  is  represented  at  Cerrig-y-druidion  by  a  series  of  grey, 
1  Quart.  Journ.  Geol.  Soc.  vol.  xxxiii.  p.  207. 
2  Ibid.  vol.  xxxv.  (1879)  p.  200. 
3  Ibid.  vol.  xxxvi.  (1880)  p.  277. 
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