Yol.  49.] 
CONGLOMERATES  IN  CAERNARVONSHIRE. 
look  like  empty  lenticles  or  shrinkage-cracks,  possibly 
dipping  towards  the  lake.  These  latter  may  possibly  be 
unconformable,  as  suggested  by  Mr.  Maw,1  especially 
as  they  are  just  about  where  a  patch  of  the  supposed 
newer  rocks  might  come  on  ;  but  the  exposures  are 
now,  at  least,  too  obscure  to  have  much  independent 
value.  The  greater  part  of  the  rest  of  the  cutting 
is  occupied  by  greenish  banded  grits  (1)  in  broken 
masses,  which  have  a  general  dip  towards  the 
E.S.E.;  they  seem  to  overlie  some  of  the  purple 
slates,  and  at  the  other  end,  after  a  fault,  certainly 
underlie  others  in  a  small  anticlinal.  Finally  we 
come  to  a  slickensided  junction,  on  the  other  side 
of  which  is  some  more  coarse  conglomerate  (m) 
like  that  at  the  other  end  of  the  cutting,  followed  by 
a  blue  grit  ( n )  in  which  the  section  ends. 
This  description  will  be  found  to  correspond  very 
closely  with  that  of  Mr.  Maw,  and  I  now  proceed  to 
discuss  its  teachings.  The  conglomerate  (a)  lies 
in  a  hollow  of  the  felsite,  hence  that  felsite  must 
have  been  exposed  when  the  conglomerate  was 
formed,  and  its  upper  surface  must  have  been  that 
which  is  now  nearly  horizontal ;  but  the  felsite 
comes  into  vertical  connexion  with  the  Purple  Slate 
(b)  with  no  conglomerate  between.  The  nature  of  the 
connexion  seems  at  first  sight  to  be  an  intrusion  on 
the  part  of  the  felsite,  for  this  runs  into  tongues  in 
the  slate.  I  now,  however,  perceive  that  this  ap¬ 
pearance  must  be  deceptive,  for  an  examination  of  a 
polished  surface  of  the  junction  shows  that  the  slate 
is  all  broken  to  fragments,  which  will  account  for  the 
semblance  of  intrusion,  the  felsite  also  being  broken. 
Put  in  spite  of  this,  it  is  seen  that  coarser  bands  in 
the  slate  must  originally  have  run  parallel  to  the 
junction — i.  e.  the  slate  must  have  been  deposited  on 
the  felsite.  At  that  time,  therefore,  the  surface, 
which  is  now  vertical,  must  have  been  nearly  hori¬ 
zontal,  and  hence  the  conglomerate  must  have  been 
deposited  at  a  later  date,  after  the  felsite  was 
turned  on  end.,  i.  e.  after  the  deposit  of  the  Purple 
Slate.  In  point  of  fact,  though  not  actually  seen 
in  contact  with  the  slate  between  a  and  c,  at  the 
latter  spot  it  has  passed  over  to  the  other  side  of  it, 
and  is  therefore  unconformable  both  to  it  and  to 
the  felsite,  provided,  of  course  (of  which  I  think 
there  can  hardly  be  a  doubt),  that  the  conglome¬ 
rates  at  a  and  c  are  parts  of  the  same  mass.  With 
this  conglomerate  go  the  grey  slates  and  grits 
(d  and  /),  in  fact  all  the  beds  in  the  synclinal 
1  Geol.  Mag.  for  1868,  p.  121. 
■Section  along  the  Llanberis  Railway  (Scale:  1  inch  =  100  yards). 
