444 
EEY.  J.  F.  BLAKE  ON  FELSITES  AND 
[Aug.  1893, 
here  shown  by  the  Bronllwyd  Grit.  In  my  former  paper  it  was 
stated  that  no  definite  proof  of  its  unconformity  could  be  given, 
though  several  things  pointed  that  way.  I  would  here  call  again 
to  remembrance  how,  in  spite  of  the  main  mass  being  limited 
to  the  east  of  a  well-marked  line,  the  rock  spreads  over  the 
surface  to  the  west,  as  shown  even  in  the  Geological  Survey  Map, 
round  Frid  Fedw,  and  at  quite  low  levels  on  either  side  of  the  River 
Ogwen  at  Bethesda,  where,  lying  to  the  west  of  Purple  Slates,  a 
careless  stratigraphy  might  put  it  below  them.  The  basal  beds 
here  exposed  have  a  coarse  slate-breccia  at  the  bottom,  wherever 
that  can  be  seen ;  then  comes  a  grit  with  small  white  quartz-pebbles ; 
and  then  some  curious  alternations  of  1-  or  2-inch  bands  of  slate 
and  grit,  with  cleavage  confined  to  the  former.  The  importance  of 
noticing  this  is  that  we  have  here  an  example  of  a  set  of  beds  which 
are  generally  accepted  as  post-Llanberis 1  and  which  certainly  have 
the  workable  Purple  Slates  underlying  them,  separated  from  the 
main  mass  and  comparable  in  position  to  those  which  are  claimed 
to  be  of  the  same  age  farther  south. 
III.  Between  Moel  Rhiw-wen  and  Moel-y-Ci. 
In  this  area  I  have  marked  on  my  map  (Quart.  Journ.  Geol.  Soc. 
vol.  xlviii.  1892,  p.  243)  a  patch  of  supposed  ‘post-Cambrian’ 
conglomerate.  The  evidence  in  this  case  is  not  of  the  clearest. 
The  mass  appears  to  have  a  much  wider  surface-distribution  than 
would  be  expected  if  it  were  a  bed  in  the  series  around  it,  and  the 
greatest  length  does  not  quite  run  in  the  expected  direction.  If 
it  were  taken  as  part  of  the  adjacent  Cambrian  Series,  its  position 
between  the  Banded  Slates  and  Lower  Purple  Slates  would  indicate 
it  to  be  part  of  the  Rhiw-wen  Grit.  The  principal  reason  for  con¬ 
sidering  it  of  later  date  is  its  close  resemblance,  in  its  irregular 
character  and  in  containing  slate-fragments,  to  the  conglomerate 
at  Llyn  Padarn,  which  latter  is  thought  to  be  proved  post-Llanberis. 
This,  of  course,  is  an  argument  which  cuts  both  ways  and  has  no 
independent  value.  We  note,  however,  that  if  the  conglomerate 
here  be  not  post-Llanberis,  then  (1)  it  is  not  connected  with  any 
felsite  ;  (2)  it  would  occur,  not  at  the  base,  but  in  the  middle  of 
the  Cambrian  Series  ;  (3)  it  would  not  be  on  the  same  horizon  as 
that  in  the  Penrhyn  Tunnel,  for  it  would  lie  below  the  Lower  Purple 
Slates  which  are  actually  here  overlain  by  the  St.  Ann’s  Grit. 
IV.  Llyn  Padaen  and  Y  Bigl. 
The  section  exposed  along  the  mineral  railway  is  so  accessible  and 
easy  to  see  that  it  has  often  been  examined  and  several  times 
described ;  and  the  supposed  structure  of  the  district  has  been 
mainly  founded  on  its  teachings.  It  will  be  necessary,  therefore, 
for  purposes  of  reference  to  give  the  section  once  more,  as  now 
interpreted. 
1  [At  the  reading  of  the  paper  these  were  called  *  post-Cambrian,’  but  the 
uglier  term  now  adopted  is  less  liable  to  lead  to  confusion. — July  15th,  1893.] 
