Yol.  49.]  FELSITES  AND  CONGLOMERATES  IN  CAERNARVONSHIRE.  441 
35.  On  the  Felsites  and  Conglomerates  between  Bethesda  and 
Llanllyfni,  North  Wales.  By  the  Bev.  J.  F.  Blake,  M.A., 
F.G.S.  (Bead  Way  10th,  1893.) 
Contexts. 
I.  Introduction  .  441 
II.  The  Penrhyn  Quarry  Tunnel  .  442 
III.  Between  Moel  Bkiw-wen  and  Moel-y-Ci  .  444 
IV.  Llyn  Padarn  and  Y  Bigl  . 444 
Y.  The  South-western  side  of  Llyn  Padarn .  450 
YI.  The  supposed  Slates  intercalated  in  the  Felsite .  456 
YII.  Bettws  Garmon  Yalley .  458 
VIII.  Moel  Tryfaen .  458 
IX.  Mynydd-y-Cilgwyn  and  Llanllyfni .  463 
X.  Distribution  of  the  Felsites  in  the  Cambrian  Series  .  464 
XI.  Age  of  the  overlying  Conglomerates  and  Grits .  465 
XII.  Concluding  Speculations  . 465 
I.  Introduction. 
A  paper  which  was  read  to  this  Society  on  December  9th,  1891, 1 
had  for  its  primary  object  the  proof  that  certain  felsites,  occurring 
in  North-west  Caernarvonshire,  which  had  been  claimed  to  be  of 
pre-Cambrian  age,  were  in  reality  part  of  the  Cambrian  succession. 
This  conclusion  had  indeed  been  previously  arrived  at  in  an  earlier 
paper,2  and  had  been  confirmed  by  Sir  Archibald  Geikie  in  his 
Presidential  Address ; 3  but  further  proofs  were  brought  forward 
from  a  study  (1)  of  the  Cambrian  succession,  (2)  of  the  felsite  itself. 
The  latter  part  of  the  paper,  however,  with  its  somewhat  startling 
assertion  of  the  ‘  post-Cambrian  ’  age  of  certain  conglomerates  which 
had  hitherto  been  considered  either  to  mark  the  base  of  the  Cambrian 
or  to  be  an  integral  portion  of  it,  required  perhaps,  considering  the 
weight  of  authority  to  which  the  statements  were  opposed,  some¬ 
what  more  argumentation  than  was  there  given  to  it.  Since  the 
date  of  that  paper  further  information  has  been  obtained,  which, 
while  it  has  necessitated  some  minor  corrections,  is  found  in  the  end 
to  greatly  strengthen  the  main  contention. 
With  regard  to  the  felsite,  or,  as  one  ought  properly  to  say,  the 
felsites,  in  spite  of  Dr.  Hicks’s  latest  assertion  that  “there  is  abun¬ 
dant  evidence  to  show  that  the  Llyn  Padarn  quartz-felsite  is  of 
pre-Cambrian  age  4,”  it  seems  to  me  like  flogging  a  dead  horse  to 
prove  any  further  their  Cambrian  age.  They  occur,  as  will  be  seen, 
at  so  many  horizons  that  they  could  only  be  one  mass  if  they  were 
intrusive ;  and  of  their  being  this,  even  where  they  appear  at  first 
sight  to  behave  as  such,  there  is  no  evidence,  but  very  much  to  the 
contrary. 
1  Quart.  Journ.  Geol.  Soc,  vol.  xlviii.  (1892)  p,  243. 
2  Ibid.  vol.  xliv.  (1888)  p.  271.  3  Ibid.  vol.  xlvii.  (1891)  Proc.  p.  90. 
4  Ibid.  vol.  xlviii.  (1892)  p.  261. 
Q.  J.  G.  S.  No.  195.  2  g 
